CBSE Class 8 Science Chapter 1 Crop Management Revision Notes Crop Management Crops

When plants of the same type are grown and cultivated in a field on a large scale, it is called a crop (fasal in Hindi). Examples include wheat, paddy, and other crops.

India is a vast country, where the climatic conditions like temperature, humidity, and rainfall vary from one region to another. Accordingly, a rich variety of crops is grown in different parts of the country.

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Despite this diversity, two broad cropping patterns can be identified particular season in which they grow.

These are as follows

1. Kharif Crops

The crops which are sown in the rainy season are called Kharif crops or summer crops, for example, paddy (rice), maize, soyabean, cotton, groundnut, millets (bajra), etc. These crops are generally grown from June to September.

” Cbse Class 8 Science Chapter 1 Notes “

2. Rabi Crops

The crops that are grown in winter season are called Rabi crops or winter crops, for example, wheat, gram, pea, mustard, and linseed.

These crops are generally grown from October to March.

Note: Paddy or rice crops are grown in the rainy season because they require a lot of water. It cannot be grown in the winter season.

Class 8 Science Chapter 1 Crop Management Agricultural Practices

Cultivation of crops involves several activities undertaken by farmers over time. activities or tasks are referred to as agricultural practices.

The various agricultural practices of crop production involve

1. Preparation of soil
2. Sowing
3. Adding manure and fertilisers
4. Irrigation
5. Protecting from weeds
6. Harvesting
7. Storage

” Crop Management Notes For Class 8 “

Class 8 Science Chapter 1 Crop Management Notes For self-study Preparation Of Soil

The first step for crop production is soil preparation. Soil is the uppermost layer of the Earth where plants are grown.

Only a few centimeters of the top layer of soil support plant growth. It provides water, air, minerals, humus, etc. to the plants. Soil is prepared for sowing seeds by three major methods, which are as follows

” Cbse Class 8 Science Crop Production Notes “

1. Ploughing

The process of loosening and turning the soil is called ploughing or tilling. It is done by using a plough. The loosening and turning of soil by ploughing is helpful because of the following Reasons

Plant ixsois can pencil ale lively mul ileepci Inin the soil and plants are belli lb inly to the ground.

Loose soil helps the plant roots to breathe easily. Ploughing of soil brings the nutrient-rich soil to the top so that the plant can use these nutrients.

Loosened soil helps in the growth of earthworms and microbes present in the soil. These organisms are friends of the farmers since they further turn and loosen the soil and add humus to it.

2. Levelling

Levelling of soil is done with the help of a leveller. The ploughed field may have big pieces of soil called crumbs. Levelling helps to break these crumbs with a plank and prevents loss of moisture from the loose soil. It also helps in the uniform distribution of water in the fields during irrigation.

3. Manuring

Continuous growing of crops makes the soil deprived in certain nutrients. Therefore, farmers have to add manure to the fields to replenish the soil with nutrients. This process is called manuring. Sometimes, manure is added to the soil before tilling. This helps in the proper mixing of manure with soil.

” Agricultural Practices Class 8 Notes “

Agricultural Implements

The above-mentioned practices are done by using various tools. These tools are called agricultural implements. These are discussed below.

Plough It is the most ancient tool used for tilling the soil, adding manure and fertilisers, removing weeds, and turning the soil. This is made up of wood and is drawn by a pair of bulls and other animals (horses, camels, etc). It comprises of strong triangular iron strip called a ploughshare

The Main part of the plough is a long log of Wood Wich is called plough shaft it has a handle at one end and the other end is attached to a beam which is placed on buld necks one pair of bulls which is placed on bull necks one pair of bulls and a man can easy operate the plough Now a day the indigenous wooden plough is inceresigly being replaced by iron plough.

It is used for removing weeds and for loosening the soil. It is simple in structure, a long rod of wood or Iron. A strong, broad, and iron. The iron plate is fixed; all of its codes will work like a blade. It is pulled by the animals.

” Kharif And Rabi Crops Class 8 “

Cultivator nowadays, ploughing is done by a tractor-driven cultivator. It is a modern form of plough which can dig into a considerable area of soil at the same time loosen it and turn it. A cultivator saves both labour and time.

” Soil Preparation For Crop Management “

1-4 Sowing

It is the process of scattering or planting seeds into the soil to grow a new crop plant. Showing is the most important part of crop production before sowing, claiming healthy and good quality of seeds are selected to produce healthy plants and high yields of crops.

Methods of Sowing

Sowing is done either manually or by using machines, as described below.

Traditional Tools

Traditional tools are used with the help of bulls. It was used by farmers earlier in their fields for sowing seeds. Its upper funnel-shaped part is filled with seeds. The seeds pass down through two or three pipes having sharp ends. These ends pierce into the soil and place seeds in that space

” Irrigation Methods In Agriculture Class 8 “

2. Seed Drill

It is a mechanical method of sowing by using a tractor. The seeds are sown uniformly at equal distance and depth. The soil is covered with soil after sowing.

This method protects the seeds from the damage caused by birds. It also saves time and labour. The advantage of sowing seeds at an appropriate distance and depth is that it prevents overcrowding of plants and each plant gets sufficient sunlight, nutrients, and water from the soil.

The National Seed Corporation (NSC) in India plays a key role in producing high-quality agricultural seeds and has contributed to the establishment of seed¬ testing laboratories across various regions in the country.

” Manure Vs Fertilisers Class 8 “

Adding Manure And Fertilisers

Manure and fertilisers are substances that are rich in nutrients and are added to the soil for the healthy growth of plants.

The addition of manure or fertiliser to the soil is called manuring. Farmers add manure to the fields to replenish the soil with nutrients. Improper or insufficient manuring makes plants weak. For replenishing soil with nutrients following methods are used.

Manures

These are organic (natural) substances derived from the decomposition of plant and animal wastes, like cow dung, etc. Plant and animal wastes are dumped in pits in open places and allowed to decompose by some microorganisms. The decomposed matter is used as organic manure.

” Types Of Crops For Cbse Class 8 “

Fertilisers

These are the chemicals that contain the necessary plant nutrients like nitrogen, phosphorus, etc. Examples- Urea, ammonium sulphate, super phosphate, potash, NPK (Nitrogen, Phosphorus, Potassium).

Fertilisers are used to increase soil fertility, which helps farmers to get better yields of crops such as wheat, paddy, and maize. The excessive use of fertilizer is harmful because it makes the soil less fertile and also causes water pollution.

Advantages of Manure Over Fertilisers

The organic manure is considered better than fertilisers for the following reasons

1. It enhances the water-holding capacity of the soil.
2. It makes the soil porous, due to which the exchange of gases becomes easy.
3. It increases the number of friendly microbes.
4. It improves the texture of the soil.

” Seed Drill And Sowing Methods Class 8 “

Crop Rotation

The practice in which different types of crops, especially leguminous and non-leguminous crops, are grown alternately in the same field is called crop rotation.

This procedure is followed to improve the fertility of the soil. For example, crops like cereals, vegetables, etc., deplete the soil of the soil. Therefore, leguminous crops are grown alternatively after these crops to maintain the nitrogen level of the soil

The leguminous plants possess nitrogen-fixing bacteria called Rhizobium in their roots, which can directly fix atmospheric nitrogen into soluble nitrogenous compounds. Rotation of crops also saves a lot of nitrogenous or chemical fertilisers.

” Sprinkler And Drip Irrigation Systems “

CBSE Class 8 Science Chapter 1 Crop Management Revision Notes Irrigation

The process of supplying water to crop plants at regular intervals is called irrigation. Water Is Important for the proper growth and development of all living organisms, including plants. A plant contains nearly 90% of water.

Importance of Water to Crop Plant

1. Water is absorbed by the plant roots. Along with water, minerals and fertilisers are also absorbed.
2. Water is essential because germination of the seed does not take place under dry conditions.
3. It also protects the crop from both frost and hot air currents.
4. For healthy crop growth, fields have to be watered regularly to maintain the moisture of the soil

Frequency and Timing of Irrigation

The frequency and timing of irrigation differ from crop to crop, soil to soil, and season to season. For example, in summer, the frequency of irrigation is required at a higher rate than in rainy or winter seasons. This is because the rate of evaporation of water from soil and leaves is high in the summer season.

” Weeding Methods In Agriculture “

Sources of Irrigation

Water supplied for irrigation can be taken from different sources, Example rain, wells, tubewells, ponds, lakes, rivers, dams, and canals

Traditional Methods of Irrigation

The water from different sources, such as wells, lakes, and canals, is lifted by different methods to take it to the fields. Cattle or human labour is used in these methods

The various traditional ways of irrigation are moat (pulley system), dhckli, chain pump, and rahat (lever system). Pumps are commonly used for lifting water. They use diesel, biogas, electricity, and solar energy

Modern Methods of Irrigation

Traditional methods use manpower and animal power, and there is a wastage of water that also occurs during irrigation. To overcome these problems, modern methods of irrigation are used. The main methods used are as follows

” Threshing And Winnowing Process “

1. Sprinkler System

In this system, there is a main pipeline laid in the field joined with perpendicular pipes, having rotating nozzles on the top.

When water is allowed to flow through the main pipe under pressure with the help of a pump, it escapes from rotating nozzles and is sprinkled like rain over the crops. Sprinklers are very useful for lawns, coffee plantations, and several other crops. This system of irrigation is useful on uneven land where sufficient water is not available.

2. Drip System

In this system, water flows through the narrow pipes and falls drop by drop just near the roots of plants.

In this method, there is no wastage of water as water falls drop by drop, therefore, this system of irrigation is very useful in regions where the availability of water is poor. It is the best technique for watering fruit plants, gardens, and trees.

” Leguminous Plants And Nitrogen Fixation “

Protection From Weeds

The unwanted, wild plants that grow along with the cultivated crops are called weeds.

1. The process of removing weeds is called weeding. It is necessary to remove weeds from time to time because
2. These unwanted plants are harmful because they compete with the crop plant for nutrients, fertiliser, space, light, and water. Thus, they reduce crop yield.
3. Some weeds interfere with even in harvesting and may be poisonous for animals and humans.

Weeding Methods

Farmers adopt many ways to remove weeds and control their growth. Tilling before sowing crops helps in uprooting and killing weeds, which may then dry up and get mixed with the soil. The best time for the removal of weeds is before they produce flowers and seeds. Weeding can be done either manually or by spraying weedicides as discussed below

Manual Weeding

In this method, the weeds are removed from the crop field by pulling them up by hand and throwing them away. Another method of manual weeding is done by using a khurpi. A seed drill is also used to uproot weeds.

” Traditional And Modern Irrigation Techniques “

Spraying Weedicides

Certain chemicals called weedicides (herbicides) like 2,4-D are sprayed in the fields to kill the weeds. The crops remain unaffected by weedicides.

It should be sprayed during the vegetative growth of weeds before flowering and seed formation, otherwise, weeds become difficult to control. Precautionary measures such as covering the nose and mouth with cloth should be taken while spraying weedicides, as they pose health hazards.

Harvesting

The process of cutting and gathering mature (ripened) crop is called harvesting. When crops get mature, they usually turn yellow, golden, or brown. In harvesting, crops are pulled out or cut close to the ground. 1 Harvesting in our country is either done manually by a sickle or by a machine called a harvester.

To obtain seed grains from harvested crops, the following two methods are used

” What Are Kharif And Rabi Crops With Examples? “

Threshing

The process by which the grain seeds are separated from the chaff is called threshing. This is carried out with the help of a machine called a combine, which is a combination ofa harvester and a thresher.

Winnowing

After threshing, the grains are separated from chaff by a process called winnowing. In this process, the mixture of grain, hay, and chaff is dropped from a height into blowing wind.

The heavier seed falls straight to the ground, whereas the chaff and hay are much lighter and are carried away by the wind. The grains form a separate heap and can be collected and packed in gunny bags.

” What Are The Advantages Of Crop Rotation? “

Harvest Festivals

After three or four months of hard work, there comes the day ofthe harvest. The sight of golden fields of standing crop, laden with grain, fills the hearts of farmers with joy and a sense of well-being.

The period is one of great joy and happiness. It is celebrated with great zeal. Special festivals associated with the harvest season are Pongal, Baisakhi, Holi, Diwali, Nabanya, and Bihu.

Storage

The harvested and cleaned grains are dried to evaporate moisture from them before storage. Since the crops/grains have to be kept for a longer time, they should be safe from moisture, insects, rats, and microorganisms. Also, insecticides, rodenticides are sprayed to protect them from rats and insects.

The grains are stored in jute bags or metallic bins by tanners. At large scale, grains are stored in silos and granaries to protect them from pests. Dried neem leaves are used for storing food grains at home, as neem is a natural insecticide

Class 8 Science Chapter 1 Crop Management Short Notes For Quick Revision Food From Animals

Besides plants, some animals also provide us with different kinds of food. The food obtained by animals is rich in protein.

The food derived from animals and the sources are tabulated below

Animal Husbandry

The process of rearing animals, by providing them proper food, shelter, and care, is called animal husbandry. Fish are an important source of animal food in coastal areas. We get cod liver oil from fish, which is rich in vitamin D

Animal husbandry Agricultural practice of breeding and raising livestock.

Agriculture practices: Tasks carried out by farmers for the cultivation of crops.

• Crop Plants of the same kind, cultivated in a field on a large scale.
• Fertilisers are Chemical substances used to increase the fertility of soil.
• Granaries are Places where a large amount of grain can be stored.
• Harvesting Process of cutting and collecting the mature crop.

” What Is The Process Of Threshing And Winnowing? “

Irrigation: Water supply to crops at regular intervals.

• Kharif Crops grown in the rainy season

Manure: Organic substances derived from the decomposition of plant and animal wastes.

• A plough Agricultural implement used for tilling or plowing. ploughing the soil.
• Rabi Crops are grown in the winter season.

Sowing: Planting the seeds in the soil.

• Seeds fertilised and matured ovules.
• Storage Process of protecting the grains, by keeping them in closed containers.
• Silo Tall and cylindrical structures used for storage of grains.
• Threshing Method of separating seed grains from the chaff.
• Winnowing Method of separating seed grains from the chaff by blowing air.
• Weeds are Undesirable plants that grow along with crops. Weedicide Chemicals used to kill weeds

CBSE Class 8 Science Chapter 12 Some Natural Phenomena: Lightning

A short burst of fire, which occurs due to the sudden flow of electric current between two objects, is known as an electric spark.

It can be observed in our daily life in many situations, when a plug is loose in its socket, when the wires become loose on an electric pole, etc. In nature, lightning is caused by the accumulation of charges in the clouds.

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Initially, people are afraid of lightning and consider it the wrath of God. But we need not be scared of lightning; we should take precautions to protect ourselves from the sparks.

CBSE Class 8 Science Chapter 12 Charging By Rubbing

When an object is rubbed with an appropriate object (like polythene, dry hair), it acquires a small electric charge, said to be a charged object. So, we can say that an electric charge can be produced by rubbing one object with another object of a specific kind. An object with no electric charge is said to be an uncharged object.

Some common examples of charging by rubbing are

1. When a plastic refill is rubbed with polythene, it acquires a small electric charge.
2. When a plastic comb is rubbed with dry hair, it acquires a small electric charge

CBSE Class 8 Science Chapter 12 Types Of Charges And Their Interaction

When two objects are rubbed together, then both the objects get charged with opposite charges, i.e. positive charge (+ sign) and negative charge (- sign) Example; When we rub a glass rod with a silk cloth, then the charge acquired by a glass rod is positive and silk cloth gets a negative charge.

The nature of the object determines which object gets a positive charge and which one gets a negative charge.

Depending on the type of electric charge on the objects, they will either attract or repel each other.

Like charges repel each other. It means, if both bodies have the same type of charges, they will repel each other.

Unlike charges attract each other. It means, if both bodies have different types of charges, they will attract each other.

The electrical charges generated by rubbing are known as static charges. Example: by rubbing a plastic comb with dry hair. Static charges do not move themselves. If these charges move, then they constitute electric current.

Transfer of Charges

Electric charge can he transferred from a charged object to an uncharged object, it is called transfer of charge. On the basis ot transfer of charges, we can construct a device called an electroscope. It is a device that is used to test whether an object is carrying a charge or not.

Detection of Charge with Electroscope

To detect the electric charge on a body, we touch the metal top ot the electroscope with it and notice the aluminium leaves.

1. If the body is electrically charged, then the leaves diverge.
2. If the body is electrically uncharged, then the leaves do not diverge.

An electroscope proves that electric charge can be transferred from a charged object to another through a metal conductor.

Earthing

The process of transferring an electric charge from a charged object to the earth is known as earthing. When a body loses charge to the earth or any other body, then it is said to be discharged.

Note: Earthing is provided in buildings to protect us from electrical shocks due to any leakage of electric current.

CBSE Class 8 Science Chapter 12 Story Of Lightning

1. Lightning is bright streaks or flashes of light seen in the dark cloud sky. Due to the transfer of charges from cloud to cloud or from cloud to the earth. It is usually followed by a loud cracking sound known as thunder.
2. During the development of a thunderstorm, there is always an upward movement of air while the water droplets move downward.
3. These vigorous movements cause the separation of charges. Therefore, the top of the cloud becomes positively charged while the bottom of the cloud becomes negatively charged.
4. The quantity of opposite electric charges on the top and bottom of storm clouds becomes very large, so the air, which is normally a poor conductor of electricity, will no longer be able to resist their flow. Therefore, the electric charges start flowing at high speed.
5. When both the positive and negative charges of the cloud meet, they produce an intense spark of electricity. We observe this electric spark as the flash of lightning in the sky. This process is also called an electric discharge.
6. The process of electric discharge can occur between two or more clouds or between clouds and the Earth.

CBSE Class 8 Science Chapter 12 Lightning Safety

During thunderstorms and lightning, a house or a building is always a safe place to be in. Any open place is not a safe place.

If you are travelling by car or by bus, then you are safe inside with the windows shut. The following precautionary measures should be taken to protect ourselves from lightning strikes during a thunderstorm.

CBSE Class 8 Science Chapter 12 Lightning Safety Outside the House

Dos During a Thunderstorm

1. We should stay away from tall trees.
2. If we are in a park or forest, we should take shelter under shorter trees.
3. If we are in a closed car, we should stay inside with the windows and doors closed.
4. Squat on the Hoot. Thin position makes us the smallest target to he struck.

Don’ts During a Thunderstorm

1. Do not stand in open ground during a lightning storm,
2. Do not carry an umbrella as its metal rod is a good conductor of electric charge.
3. Always stay away from poles or other metallic objects.

Lightning Safety Inside the House

Dos During a Thunderstorm

1. We should switch OFF the TV as antennas fixed on the roof of a building are prone to lightning strikes,
2. We should avoid touching the metallic pipes and electric equipment connected by electric wires in the house.
3. We should unplug electrical appliances like the refrigerator, computers, etc.

Don’ts During a Thunderstorm

1. We should not use a wired telephone,
2. We should not take a bath as water is a good conductor of electricity.

Lightning Conductor

A device that is used to protect buildings from the effects of lightning is known as a lightning conductor.

Lightning Conductor

A device that is used to protect buildings from the effects of lightning is known as a lightning conductor. end of the lightning or is fixed to a vital plate, which is deep into the pound, is shown in the figure.

Lightning Conductor Protects the Building From Thunderstorms

1. If the lightning strikes on building, it will first strike the top end of the lightning conductor.
2. The lightning conductor provides  an easy route for the runnier of electrical charge to discharge to ground,
3. The electrical energy of lightning, which passes through a tin metal strip, gel discharged safely into the ground, and the hurled metal plate.
4. Since the lightning charge does not pass through the Imilding, no damage is caused.

CBSE Class 8 Science Chapter 12 Earthquake

A sudden shaking or trembling of the earth that lasts for a very short time is an earthquake. It can cause immense damage to buildings, bridges, dams, and people.

A sudden shaking of the earth causes tremors, and these tremors produce waves on the surface of the earth. Tremors can be caused by to volcanic eruption or a meteor hitting the Earth.

Causes of Earthquakes

The tremors are caused by the disturbances inside the Earth. Before learning about the causes, we should know that the Earth consists of three layers.

1. Core The innermost part of the Earth is called its core.
2. Mantle The central region of the Earth is called the mantle.
3. Crust The outermost layer of the Earth is called the crust.

The outermost layer ofthe most is fragmented, due to which the Earth’s crust is divided into many plates. These plates are in Continuous motion

When these plates collide with one another or brush past one another then they cause disturbance in the Earth’s crust. This disturbance results from an earthquake on the Earth’s surface.

The place inside the Earth’s crust where the earthquake is generated is called the focus of the earthquake.

Seismic or Fault Zones

As earthquakes are caused by the movement of plates, the boundaries of the plates are the weak zones where earthquakes are more likely to occur. These weak zones are also known as seismic or fault zones. In India, Kashmir, Western and Central Himalayas, the whole of North-East, Rann of Kutch, Rajasthan, and the Indo-Gangetic plain are the most threatened areas.

Richter Scale

It is a scale that is used to measure the intensity or magnitude of an earthquake. Destructive earthquakes have magnitudes higher than 7 on the Richter scale.

Seismograph

The seismic waves are those waves that are produced by tremors on the Earth’s surface. These waves are recorded by an instrument called the seismograph.

• This instrument is simply a vibrating rod or a pendulum that starts vibrating when tremors occur. A pen is attached to the vibrating system, and this pen records the seismic waves on paper that moves under it.
• By studying waves, scientists can construct a complete map of the earthquake, as shown in the figure. They can also calculate its power to cause destruction.

Protections Against Earthquake

Some of the important precautions that can be taken by people living in seismic zones are as follows

1. All the quake-proof houses should be constructed using modern building technology. For which, we should consult a qualified architect and structural engineers.
2. Even in highly seismic areas, it is advisable to use mud or timber for building houses, as these are better than heavy materials, which keep roofs as light as possible.
3. There should be a tight fixation of cupboards and shelves to the wall, so that they do not fall when shaking occurs.
4. The heavy mirrors, photo frames, water heaters, etc., should be mounted to the wall at low levels, so that in the event of an earthquake, they do not fall on the people.
5. Since some buildings may catch fire due to an earthquake, it is necessary that all buildings, especially tall buildings, have firefighting equipment in working order.

In the event of an earthquake, follow these steps to protect yourself.

If you are at home

1. Try to get a shelter under a table and stay there till the shaking stops.
2. Stay away from tall and heavy objects as they may fall on you.
3. If you are in bed. Then do not get up. Immediately, protect your head with a pillow.

If you are outside your home

1. Observe a clear spot away from buildings, trees, and overhead power lines. Drop to the ground.
2. Besides this, if we are in a car or a bus, we should not get out. The car should be driven slowly to a clear spot.

CBSE Chapter 3 Coal And Petroleum Natural Resources

The various resources which are obtained from nature are called natural resources, for example, air, water, soil, plants, petroleum, etc. These are necessary for the survival of all living beings. Based on their availability in nature, natural resources can be classified into the following two types

1. Inexhaustible Natural Resources

Those natural resources which are present in unlimited quantity in nature and are not likely to be exhausted (used up) by human activities are called inexhaustible natural resources, Example; sunlight, air, water, etc.

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2. Exhaustible Natural Resources

Those natural resources which are present in a limited quantity in nature and can be exhausted (used up) by human activities are called exhaustible natural resources, for example, coal, petroleum, natural gas, minerals, forests, wildlife, etc. Coal, petroleum and natural gas are collectively known as fossil fuels. As they were formed from the remains of dead organisms (fossils).

Class 8 Science Chapter 3 Coal And Petroleum Revision Notes Coal

It is a hard, black combustible fuel. Coal is found in deep coal mines under the surface of the Earth. It burns in the presence of air and produces CO2, gas and heat. Earlier, it was used in railway engines to produce steam to run the engines.

Formation Of Coal

About 300 million years ago, the Earth had dense forests in low-lying wetland areas. Natural processes like flooding, earthquakes, volcanoes, etc., buried these forests under the surface of the Earth.

As more soil deposited over them, they were compressed. The temperature also rose as they sank deeper and deeper. Under high pressure and temperature dead plants got slowly converted into coal.

This slow process of conversion of dead vegetation buried deep under the Earth into coal is called carbonisation.

Coal As A Source Of Energy

Coal contains mainly carbon, and when heated in air, it burns and produces mainly carbon dioxide gas and heat. The reaction can be written as

Carbon Oxygen Carbon dioxide Heat
(Coal) (From air)

Uses of Coal

Some of the uses of coal are given below

1. Coal is one of the fuels used to cook food.
2. It is used in thermal power plants to produce electricity.
3. It is used as a fuel in various industries.
4. It is used for the manufacture of coke, coal tar and coal gas.
5. Products Obtained from Coal
6. Coal is processed in the industry to get some useful products. Which are coke, coal tar and coal gas.

1. Coke

It is a tough, porous and black substance. Coke is an almost pure form of carbon. When coal is heated in the absence of air, then coal gas and coal tar are eliminated and ok is left behind as a black residue.

Uses of Coke

• Coke is mainly used
• In the extraction of metals such as iron, zinc, etc.
• In the manufacture of steel.
• As a fuel

2. Coal Tar

It is a thick, black liquid having an unpleasant smell. It is a mixture ofabout 200 different carbon compounds.

Uses of Coal Tar

Some of the uses of coal tar are as follows

1. The various compounds obtained from coal tar are used as starting materials for manufacturing substances used in everyday life and in industries. For example, perfumes, photographic materials, roofing materials, drugs, explosives, plastics and paints etc.
2. Naphthalene balls are obtained from coal tar and are used to repel moths and other insects.
3. Coal tar has been used for metalling the roads. But, these days bitumen (a petroleum product) is being used for metalling the roads.

3. Coal Gas

It is a gaseous fuel which is obtained during the processing of coal in the absence of oxygen to get coke. It is mainly a mixture of methane and hydrogen with some carbon monoxide.

Uses of Coal Gas

1. Some of the uses of coal gas are as follows
2. It is used as a fuel in many industries situated near the coal processing plants.
3. It has also been used for lighting purposes.
4. Science Insight Coal gas was used for street lighting for the first time in London in 1810 and in New York around 1820. This is because when it burns, it produces a good amount of light. Nowadays, it is used as a source of heat rather than light.

CBSE Class 8 Science Chapter 3 Coal And Petroleum Notes For Competitive Exams Petroleum

It is a dark coloured, thick crude (raw) oil having an unpleasant odour. The name petroleum means ‘rock oil (petra = rock, oleum = oil). It is found deep below the Earth’s crust, trapped in rocks in certain areas.

Petrol and diesel are obtained from petroleum. Petrol is used as a fuel in light automobiles, for example, motorcycles, scooters and cars, whereas diesel is used in heavy motor vehicles, for example, trucks and tractors.

Formation of Petroleum

Petroleum was formed by the decomposition of the remains of plants and animals buried under the sea millions of years ago.

The plants and animals which lived in the sea. died and their bodies settled at the bottom of the sea. The lead remains covered with mud, sand and clay. Due to high temperature, high pressure and the absence of air, these were slowly converted into petroleum and natural gas as shown in the figure.

The layer containing petroleum oil and gas is found above the water. It is because oil and gas are lighter than water and do not mix with it.

Petroleum is extracted by drilling holes (oil wells) in the Earth’s crust. In the drilling process, natural gas comes out first with a great pressure and for a time, the crude petroleum oil comes out by itself due to gas pressure.

The world’s first oil well was drilled in Pennsylvania, USA, in 1859. In 1867, oil was struck at Makum in Assam. In India, oil is found in Assam, Gujarat, Mumbai (offshore area) and the river basins of the Godavari and Krishna rivers.

Refining of Petroleum

Petroleum is a mixture of petroleum gas, petrol, diesel, lubricating oil, paraffin wax, etc.

Before the use of petroleum for specific purposes, it has to be refined (or purified). The process ofseparating the various useful constituents (fractions) of petroleum is known as refining. It is carried out in a petroleum refinery.

Various Constituents of Petroleum and Their Uses

The various constituents of petroleum and their uses are given below in the table.

Petrochemicals

The useful chemicals which are obtained from petroleum and natural gas are called petrochemicals. They are very important as these are used to manufacture a wide range of useful materials such as detergents, fibres (polyester, nylon, acrylic, etc), plastics (polythene, etc). Hydrogen gas obtained from natural gas is used in the production of fertilisers such as urea. Thus, due to its great commercial importance, petroleum is also called ‘black gold’.

Natural Gas

It mainly consists of methane (95%). Other constituents are ethane and propane. It occurs deep under the Earth’s crust either alone or along with oil above the petroleum.

In India, natural gas has been found in Tripura, Rajasthan, Maharashtra and in the Krishna Godavari delta.

It is a very important fossil fuel as it is easy to transport through pipes. When it is compressed by applying pressure, it is called Compressed Natural Gas (CNG).

This gas is used for power generation. It is now being used as a fuel for transport vehicles as it is a clean fuel.

Natural gas is used as a starting material to manufacture many chemicals (petrochemicals) and fertilisers.

CBSE Class 8 Science Chapter 3 Coal And Petroleum Notes For Quick Learning: Conservation Of Natural Resources

Coal, petroleum and natural gas are fossil fuels and will last only a few hundred years. Fossil fuels, forests and minerals, etc., are exhaustible natural resources. If consumed at a rate faster than the rate at which these are formed in nature, they will get exhausted very soon.

Once the present stock of these gets exhausted, no new supplies of these fossil fuels will be available to us shortly. This is because it requires millions of years to convert the dead organisms into these fuels.

Moreover, the burning of these fuels is a major cause of air pollution. Their use is also linked to global warming. So, we should use these fuels only when necessary as

1. It will reduce air pollution and thus lead to a better environment.
2. It will reduce the risk of global warming.
3. It will ensure their availability for a longer period.
4. How to Save Petrol or Diesel while Driving?
5. Petrol and diesel are the main fuels used for driving vehicles. So, we should make every effort to save these precious fuels.
6. In India, the Petroleum Conservation Research Association (PCRA) advises people how to save petrol or diesel while driving their vehicles.
7. The various tips to avoid the wastage of these fuels while driving are as follows:
8. Drive at a constant and moderate speed as far as possible.
9. Switch off the engine of the vehicle at traffic lights or at a place where you have to wait.
10. Ensure the correct tyre pressure of the vehicle. Low tyre pressure consumes more fuel.
11. Ensure regular maintenance of the vehicle.

Coal is A fossil fuel obtained from deep under the Earth’s crust.

Coal gas is obtained during the processing of coal to get coke. It can be used as fuel.

Coke, A tough, porous and black substance, almost pure form of carbon.

Coal, A black, thick liquid obtained by processing coal.

Fossil fuels are Fuels obtained from the decomposition of dead remains of plants and animals, for Example, petrol, coal, etc.

Natural gas is a fossil fuel which mainly contains methane and can be transported through pipes.

Petroleum is a dark coloured crude oil having an unpleasant odour.

Petroleum refining: Petroleum is separated into different fractions by this process.

Petrochemicals are useful chemicals which are obtained from petroleum and natural gas.

CBSE Class 8 Science Chapter 11 Chemical Effects Of Electric Current Conduction Of Electricity In Liquids

Like solids, liquids may also conduct electricity. Some liquids are good conductors of electricity, and some are poor conductors of electricity.

To examine whether a liquid permits electric current to pass through it or not, we can use the tester. When the liquid like lemon juice, vinegar etc., is between the two ends of the tester, the bulb glows.

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On the other hand, when the liquid, like distilled water, is between the two ends of the tester, the bulb does not glow.

From the above experiments, we can say that lemon juice and vinegar are the liquids that allow electricity to pass through them.

i.e. They are conducting liquids, while distilled water does not allow the electricity to pass through it, i.e. it is not a conducting liquid.

CBSE Class 8 Science Chapter 11 Detecting Weak Electric Current Through A Liquid

In some situations, even if the liquid is conducting but the bulb does not glow.

It may be due to the low conductivity of the liquid that it allows only a weak current to pass through it. This weak current is not sufficient to heat the filament of the bulb to a high temperature.

If the electrical conductivity of liquid which we are testing, is very small, then the current in the circuit may be weak. The weak electric current flowing through liquids can be detected in two ways.

With the help of LED (Light Emitting Diode)

With the help of a compass surrounded by turns of circuit wire.

By using LED, LED can be used in the tester in place of a bulb. It lights up even when a very weak current passes through the circuit.

l.F.n has two wires, one wire is longer than the other. The longer wire is connected to the positive terminal, and the shorter wire is connected to the negative terminal of the battery.

With the help of a compass We know that, electric current produces a magnetic effect. So, when a weak electric current flow’s in a wire, this produces a magnetic effect due to w’hich a compass needle starts deflecting. So, this test circuit shows the presence ofelectric current.

Poor Conductors can be Good Conductors

Poor conductors are those materials which allow electricity to pass through them under certain conditions.

As we know, air is poor conductor of electricity but during lightning, an electric current pass through it. Similarly, distilled water does not conduct electricity but when we add some salts, acids, bases to it, this solution starts conducting electricity.

So, we can say that liquids like solutions of acids, bases and salts are good conductors ofelectricity. The water that we get from sources like handpumps, taps, tube wells, etc., contains a small amount of natural salts. These natural salts make it a good conductor of electricity.

CBSE Class 8 Science Chapter 11 Chemical Effects Of Electric Current

When electric current is passed through conducting liquids, it causes chemical changes in the liquids. This is called the chemical effect of electric current.

The passage of an electric current through a conducting solution may cause the following chemical effects

1. Change in colour of the conducting liquid.
2. Deposits of metal on the electrodes.
3. Bubbles of gas near the electrodes.

Electrodes

The conducting rods are generally made up of carbon. These rods are known as electrodes.

The electrode connected to the positive terminal of the battery is called the anode, while the electrode connected to the negative terminal of the battery is called the cathode.

The chemical effect produced by an electric current depends on the nature of the conducting solution and the nature of the electrode used.

When electric current is passed through conducting water, it breaks the water into hydrogen gas and oxygen gas.

This breaking of water through electric current is called chemical decomposition. Such chemical decomposition of conducting liquids through electric current is called electrolysis.

CBSE Class 8 Science Chapter 11 Electroplating

The process of depositing a thin layer of desired metal over a metal object with the help of electric current is called electroplating.

Example: Wheel rims, handle of a bicycle are made shiny by coating them with chromium metal. It is one of the most common applications of the chemical effects of electric current.

The objective of electroplating is

1. To protect metal from rusting or corrosion.
2. To make metal look shinier and attractive.

Note: Carbon is only a non-metal, which can be electroplated with copper.

Process Of Electroplating

1. When an electric current is passed through the copper sulphate solution, the solution dissociates into copper and sulphate.
2. Now, the free copper gets drawn to the electrode connected to the negative terminal of the battery and gets deposited on it.
3. An equal amount of copper gets dissolved into the solution from the other electrode (Copper plate).
4. Hence, the loss of copper from the solution is restored and the process of transferring copper from one electrode to other, continues.
5. Here, the copper transfers from anode to cathode.

Uses of Electroplating

Chromium has a shiny appearance, and it does not corrode and resists scratches. So, the objects made from a cheaper metal get a coating of chromium over them.

The following are the uses of electroplating in our day-to-day activities.

1. The electroplating of silver and gold on the less expensive metals like iron, copper, etc is done by jewellery makers. So, these kind of ornaments have the appearance of silver or gold but arc much less expensive.
2. Tin cans used for storing food are made by electroplating tin onto iron. Tin is less reactive than iron. Thus, by electroplating the iron with tin, the food does not come into contact with iron directly, so it is protected from getting spoiled.

Some Major Concerns of Electroplating

The conducting solutions like copper sulphate in the electroplating process are the pollutants for environment. It contains various types of salts which may be poisonous.

It may affect aquatic as well as human life, so the disposal of the used conducting solution is a major concern for us.

Good Conductors: Materials which allow an electric current to pass through it is known as good conductors. Examples: Salt water.

Poor Conductor: Materials which allow only a very small amount of current to pass through them are known as poor conductors.

Examples: Acetic acid /vinegar.

Electrolyte: A liquid which conducts electricity is known as an electrolyte.

Electrode: The carbon rods whose metal caps are covered by copper wires and joined to a battery are known as electrode.

Electrolytic cell: An arrangement in which a cell having two electrodes is kept in a conducting liquid in a vessel is called an electrolytic cell.

Electrolysis: The chemical decomposition produced by passing an electric current through an electrolyte is called electrolysis.

Electroplating: The process of depositing a thin layer of desired metal over a metal object with the help of electric current is called electroplating.

CBSE Class Class 8 Science Chapter 10 Sound: Sound Is Produced By A Vibrating Body

Sound is produced by the to-and-fro motion of an object, also known as vibration. It means that when any object vibrates, it produces sound.

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Example: When you strike a metal, it produces sound. Similarly, musical instruments, like manjira, ghatam, kartal, etc., produce sound due to vibration.

Some musical instruments and their vibrating parts are given below

1. In veena, sitar, and guitar, the plucking of strings produces sound.
2. In a flute, a vibrating column produces sound.
3. In a tabla and mridangam, a vibrating membrane produces sound.
4. If these objects stop vibrating, then they also stop producing sound. We can even see and feel these vibrations.

To see these vibrations, strike the edge of a metal dish full of water.

You will see vibrations in the water in the form of water waves.

But in some cases, the amplitude of vibrations is so small that we cannot see them.

CBSE Class Class 8 Science Chapter 10 Sound Produced By Humans

Human produces sound with the help of the voice box, also known as the larynx. To locate the voice box in your throat, put your fingers on your throat and feel a hard bump. You can feel vibration in this part while speaking.

1. It is present at the upper end of the windpipe in the throat region.
2. It contains two vocal cords. There is a small gap between these two vocal cords known as slit.

1. Muscles attached to the vocal cords can make the cords tight or loose.
2. When lungs force air through the slit, then vocal cords vibrate and produce sound.
3. By stretching and relaxing the vocal cords, we can change our voice. When vocal cords are stretched, they produce high-frequency sound, and when they are contracted, they produce low-frequency sound.
4. The voices of men, women, and children are different because the size of the vocal cords varies among them.
5. Males have vocal cords about 20 mm long and have a low-pitched voice. Females have shorter vocal cords, about 15 mm long, and have a high-pitched voice.

Children have very short vocal cords and have a different voice

Note: Vocal cords and voice box are not the same. These are two different things. Vocal cords are parts of the voice box. These are a pair of bands of smooth muscle tissue.

Sound Needs a Medium for Propagation.

The path through which sound travels is called a medium, it can be solid, liquid, or gas. Sound needs a medium to travel, therefore it cannot travel in a vacuum. Let us take some more examples to see the propagation of sound through a medium.

When we scratch on the table, we hear some kind of sound. It shows sound can travel through a solid.

In the case of a toy telephone, sound travels through the string.

Science Insight

There is no atmosphere on the moon (i.e., no medium through which sound can travel), so astronauts use special communicators on the moon to talk to each other.

We Hear Sound Through Our Ears

We hear sound through our ears. A very simple diagram of the human ear is shown in the figure given below.

The human ear is divided into three parts

Outer ear The shape of the outer ear is like a funnel. When the sound enters in it, then it travels through a canal called the ear canal. At the end of the ear canal, there is a thin elastic, and stretched membrane, called the eardrum.

The middle ear helps to pass the sound vibrations from the eardrum to the inner ear.

Inner ear The inner ear receives the vibrations from the middle ear and changes them into signals or nerve impulses. These signal goes to the brain and we hear the sound.

CBSE Class Class 8 Science Chapter 10 Sound Amplitude, Period, And Frequency Of Vibration

Oscillation: If an object moves back and forth continuously, then the object is said to be oscillating or vibrating, and the motion is known as oscillation. Example: Vibrating simple pendulum.

The time taken by a vibrating object to complete one vibration or oscillation is called a period. It is measured in seconds.

Frequency: The number of oscillations per second is called the frequency, and it is measured in hertz (Hz).

Note Frequency is the reciprocal of the period, i.e., \(f=\frac{1}{T}\)

Amplitude: The maximum displacement of a vibrating object from its central position is called amplitude.

Amplitude and frequency are two important properties of sound, which enable us to differentiate between two sounds.

CBSE Class Class 8 Science Chapter 10 Sound Characteristics of Sound

There are mainly two characteristics of sound: Loudness. It is the propagation of sound that is used to differentiate between loud and faint sounds.

The unit of measuring loudness is decibels (dB). It is proportional to the square of the amplitude of the vibration producing the sound. It means the loudness of sound depends upon its amplitude.

When the tire amplitude of vibration is large, the sound produced is loud. When the amplitude is small, the sound produced is feeble.

If loudness is more than 80dB, then it becomes physically painful.

Pitch or Shirriness. It is the property of sound that is determined by the frequency of the vibration producing it.

Pitch is proportional to the frequency of vibration. The higher the frequency, the higher be pitch. Similarly lower the frequency, the lower be pitch.

Example: The Frequency of the drum is low, whereas the frequency of the whistle is high. Similarly, a lion makes a low-pitched sound, whereas a bird makes a high-pitched sound.

CBSE Class Class 8 Science Chapter 10 Sound – Audible And Inaudible Sounds

Every vibration produces sound, but we cannot detect sound produced beyond a certain limit of vibration. This limit is 20 Hz to 20 kHz.

Sound with a frequency between 20 Hz to 20 kHz is known as audible sound.

Sound with a frequency less than 20 1 1/, or more than 20 KHz, is known as inaudible sound.

Dogs can hear. That’s why police me high-frequency whistles which dogs can hear but humans cannot. Similarly, ultrasound equipment also works on a higher frequency, more than 20000 Hz.

Science Insight Sound having a frequency more than 20000 Hz is known as ultrasonic waves, and sound having frequency less than 20 Hz is known as infrasonic waves.

CBSE Class Class 8 Science Chapter 10 Sound – Noise And Music

1. Any unpleasant sound that causes discomfort to us is known as noise. Example: Sound of construction work, the sound of the horn, etc.
2. Any sound that is pleasant to your ear is known as a musical sound. Example: Sound produced by a harmonium, sound produced by the string of a sitar, etc.

Noise Pollution

The presence of excessive or unwanted sounds in the environment is called noise pollution. The sounds of vehicles, explosions, including bursting of crackers, machines, loudspeakers, etc., are the major causes of noise pollution.

The major sources of noise pollution in homes are television and transistor radio at high volumes, and some kitchen appliances.

Harms of Noise Pollution

Presence of excessive noise in the surroundings can cause many health-related problems like lack of sleep, hypertension (high blood pressure), anxiety, irritation and headache, and many more health disorders.

A person who is exposed to a loud sound continuously may experience temporary or even permanent impairment of hearing.

Hearing Impairment

It is the inability ofan individual to hear sounds properly. As some people are born with poor hearing ability, such of person is said to be deaf.

Often, deaf persons use a sign language with their hands to communicate with others effectively. Partial hearing loss is generally due to ear infection, injury, or old age. A person having partial hearing loss cannot hear sounds.

Measures to Limit Noise Pollution

To control noise, we must control the sources of noise. We can control noise pollution by following the points

1. Installation of silencing devices in aircraft engines, transport vehicles, industrial machines, and home appliances.
2. The industries that produce noise should be set up away from residential areas.
3. Minimise the use of automobile horns.
4. The sound of the TV and the music system should be low.
5. Plant trees along the roads and around the buildings to cut down the sounds reaching the residents.

CBSE Class 8 Science Chapter 9 Friction Notes

Friction force is defined as the force that opposes the relative motion between two surfaces of objects when they come into contact.

The direction of the friction force is always opposite to the direction of motion of the moving body.

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Types Of Friction Class 8 CBSE With Examples Spring Balance

It is the device which is used for measuring the force acting on an object. It consists of a coiled spring which gets stretched when a force is applied to it. Stretching of the spring is measured by a pointer moving on a graduated scale. The reading on the scale gives the magnitude of the force.

Factors Affecting Friction

The friction force between two surfaces depends on two factors

1. The nature of two surfaces.
2. The force with which the two surfaces are pressed together

Friction depends on the Nature of the Two Surfaces.

The force of friction is not the same for all surfaces. It depends on the roughness or smoothness of surfaces which are in contact.

When the contact surfaces are smooth, then the friction is veryr less. As the roughness increases, the friction also increases.

Friction Depends on the Force with which the Two Surfaces are Pressed Together

Friction is caused by interlocking of irregularities on the two surfaces of contact. So, even those surfaces which appear very smooth, have a large number of minute irregularities on them.

When two surfaces of objects are pressed together with greater force, it will increase the interlocking of irregularities. Thus, the friction force increases. This will become clearer from the following figure.

How To Reduce Friction Class 8 Science Methods, Types Of Friction

There are generally three types of friction, which are stated below

1. Static Friction

It is the friction that acts between two surfaces when the object starts moving from its state of rest. It is more powerful than other forces. For example, A boy pushes a heavy box, but the box does not move, then he applies more force to the box and the box just tends to start moving. At that instant, the frictional force between the floor and the box is the static friction.

2. Sliding Friction

It is the friction that acts between two surfaces when the object is already in motion or sliding over another surface, Example, skating on snow.

During sliding, an object does not get enough time to lock into the contact points on the floor.

3. Rolling Friction

• It is the friction which comes into play when a body rolls over the surface of another body.
• Example. Wheels of a skateboard roll over a road.
• The direction of rolling friction is opposite to the direction of turning of wheels.

Note: Sliding friction is smaller than static friction but greater than rolling friction.

Friction as a Necessary Evil Class 8 Explanation Friction: A Necessary Evil

The force of friction plays an important role in our day-to-day activities. Sometimes, friction is useful while in some cases, friction is harmful.

Let us learn some advantages and disadvantages of friction.

Advantages of Friction

1. Friction enables us to walk easily on the ground When we push the ground backwards while walking, the friction force pushes the foot forward. It prevents us from slipping back.
2. Friction enables us to fix the nail into the wall When we hammer a nail into the wall, the force of friction between nail and wall holds the nail tightly in the wall. Thus, we can fix nails in the wall.
3. The brakes slow down or stop moving bicycle due to friction If we press the brake lever to apply brakes, the brake pads rub the rims of bicycle wheels. The friction between them slows down the motion of bicycle.

Disadvantages Of Friction

In some cases, friction is harmful to us and can be considered an evil because of the following disadvantages

Wearing away of soles of our shoes takes place due to friction There is a friction between soles of shoes and road during walking. Due to which the soles rub against the road and the tiny pieces of soles keep on breaking off slowly.

Heat is produced due to friction. When we rub our hands together quickly for a few seconds, they become warm because friction between the hands produces heat. Similarly, when we operate a mixer and a grinder lot few minutes, its at becomes hot due to friction.

The efficiency of machines gels reduced due to friction In order to run a machine, some energy is supplied to it. Hue to friction between the parts of machine, some of the energy is wasted in the form of heat. This reduces the efficiency of machines.

Fluid Friction and Drag Force Class 8 Science Methods Of Increasing Friction

The grooves are made on the soles of shoes to increase friction.

These grooves increase the friction with the ground and the shoes get a better grip even on a slippery ground.

The tyres of vehicles are made up of treads to increase friction. A series of patterns made on the surface of tyre is called tread. The tyres are designed with treads which provide a better grip with the ground even on slippery road.

Kabaddi players usually rub their hands with dry soil to increase friction and to get a better grip on their opponents. Gymnasts apply some coarse substance on their hands to increase friction for a better grip.

Friction and Its Effects Class 8 Short Notes: Methods Of Reducing Friction

1. With the help ol polishing (lie surface We sprinkle line powder on the carrotn board and then wc polish its surface to make it smooth, so that the striker slides easily on the surface.
2. By applying lubricants (like grease or oil) to the rubbing surfaces The substances which reduce friction are called lubricants, e.g. oil , grease, etc.

When the oil, grease or graphite is applied between the moving part of a machine , a thin layer is formed there and moving surfaces do not directly rub against each other.

So, interlocking of irregularities is avoided to a greater extent, thus it reduces friction and the movement becomes easier.

Wheels Reduce Friction

It requires a lot of force or energy to move a heavy bag on the ground as the sliding friction is very large.

However, if we attach wheels or rollers to this heavy bag, then it experiences rolling friction.

In our daily routine, we often observe objects, like trolley bags and attachable fitted with rollers.

It is easier to pull the heavy bag with the attached wheels because rolling friction is smaller than sliding friction.

Real-Life Examples Of Friction For Class 8 Students: Ball Bearing

These are designed to allow moving parts of machines to roll over each other. It consists of metal balls fitted in between metal rings. In bicycles, motorbikes, etc., the ball bearing is inserted between the hub of the wheel and the axle.

When the wheel revolves, the balls of the ball bearing roll, and this reduces the friction. Hence, the wheel rolls smoothly over the axle

Advantages And Disadvantages Of Friction Class 8 Fluid Friction

Those substances which can flow easily are called fluids: liquids and gases. There is a friction whenever an object moves through a fluid, is called fluid friction.

Air and water both exert a friction force when objects move through them.

Note: The frictional force exerted by fluids is also known as drag.

The frictional force exerted by fluid on an object moving through it depends on

1. Speed of an object relative to a fluid
2. Shape of object
3. Nature of fluid (viscosity)

Disadvantages of Fluid Friction

1. A fluid friction opposes the motion of the object, which reduces its speed.
2. Fluid friction reduces the efficiency of an object. Some energy gets lost in overcoming the fluid friction.

Methods Of Reducing Fluid Friction

The fluid friction can be decreased by giving a special (streamlined) shape to the objects.

Scientists got the idea for streamline shape from the nature. The body shape of birds and fishes help them to reduce frictional force of fluids and make them to move easily.

Note The body shape that offers a small resistance to the flow of fluid around it, is called streamlined shape.

In the figure, we can see that the bird and aeroplane both have a streamlined shape in the middle with two wings and a tall tail. Fishes are also streamlined by nature due to which they move easily In the water.

Force of friction: The force that opposes the relative motion between two surfaces of the object when they come in contact.

Static friction: The friction that acts between two surfaces when the object starts moving from its rest state.

Sliding friction: The friction that acts between two surfaces when the object is already in motion.

Rolling friction: The friction that acts between two surfaces when a body rolls over the surface of another body.

Drag: The frictional force exerted by fluids is called drag.

CBSE Class 8 Science Chapter 4 Combustion And Flame Detailed Notes Combustion

A chemical process in which a substance reacts with oxygen to give off heat is called combustion. The substance that undergoes combustion is said to be a combustible substance.

It is also called a fuel which can be solid, liquid, or gas. Sometimes, light is also given off during combustion, either as a flame or as a glow. For example, wood burns by producing a flame, but charcoal burns by creating light in the form of a glow.

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Fuel For Our Body

Food is a fuel for our bodies. During respiration, the digested food (glucose) is broken down by reaction with oxygen, producing carbon dioxide, water and heat. This heat energy is utilised by our body. Thus, respiration is a type of slow combustion of food.

Combustible and Non-Combustible Substances

Not all substances around us burn. Based on this, substances are divided into two types.

Combustible Substances The substances which burn in the presence of air or oxygen are called combustible substances. Example. petrol, LPG, kerosene, etc.

Non-combustible Substances The substances which do not burn in the presence of air or oxygen are called non-combustible substances. Example. Water, glass, sand, etc.

Necessary Conditions for Combustion

There are three necessary conditions for combustion. These are as follows

1. Presence of a Combustible Substance

The presence of a substance like petrol, LPG, wood, coal, etc. is necessary for the combustion process.

2. Presence of a Supporter of Combustion

A substance which helps in combustion is called a supporter of combustion, for Example. Oxygen (or air).. For example, when the clothes of a person catch fire, the person is covered with a blanket to extinguish the fire.

The blanket cuts off the supply of air, which is needed for the burning of clothes. This shows that air is necessary for combustion.

3. Ignition Temperature

The lowest temperature at which a substance catches fire and starts burning is called its ignition temperature.

Before a substance catches fire and starts burning, it must be heated to a certain minimum temperature by supplying heat. A combustible substance cannot catch fire if its temperature is lower than its ignition temperature.

For example, a matchstick does not catch fire on its own at room temperature. When matchstick rubbed on the rough side of a matchbox the heat produced increases its temperature to ignition temperature and its start burning.

History of Matchsticks

More than five thousand years ago, small pieces of pinewood dipped in sulphur were used as matches in ancient Egypt.

A mixture of antimony tri-sulphide, potassium chlorate and white phosphorus with some glue and starch was applied on the head of a match made of suitable wood.

These days, the head of the safety match contains only antimony trisulfide and potassium chlorate. When the match is struck against the rubbing surface, some red phosphorus gets converted into white phosphorus.

This immediately reacts with potassium chlorate in the matchstick head to produce enough heat to ignite antimony trisulfide and start the combustion.

Inflammable Substances

The substances which have very low ignition temperatures and can catch fire easily with a flame are called flammable substances. Example. petrol, alcohol, Liquefied Petroleum Gas (LPG), etc.

CBSE Class 8 Science Chapter 4 How Can Fire Be Controlled?

We have learnt that there are three essential requirements for producing fire. These are fuel (combustible substance), a supporter of combustion (i.e., air or oxygen), and heat (to raise the temperature of the fuel beyond the ignition temperature).

Fire can be controlled in three ways given below

1. By removing the fuel, but in most cases, fuel cannot be eliminated.
2. By removing the heat to bring down the temperature.
3. By cutting off the air supply to the burning substances.

Fire Extinguishers

Substances that are used to cut off the supply of oxygen to the fire are known as fire extinguishers, for example, water and carbon dioxide. etc

The most common fire extinguisher is water, but it works only when things like wood and paper are on fire. It extinguishes fire by cooling the burning substances. However, water cannot be used for extinguishing fire in electrical types of equipment.

Water conducts electricity, hence, it can cause an electrical shock if electrical equipment is on fire. Water is also not suitable for fires involving oil and petrol because it sinks below the oil, and oil keeps burning on the top.

For fires involving electrical equipment and flammable substances like petrol, carbon dioxide (CO2) is the best extinguisher.

CO2 is denser than air and thus forms a layer around the burning substances, i.e. it covers the fire like a blanket, due to which oxygen cannot reach the burning substances and the fire is controlled.

Carbon dioxide can be stored as a liquid at high pressure in cylinders (as shown in the figure below). It expands enormously in volume when released from the cylinder and cools down.

Another way to obtain CO2 is to release a large amount of powdered baking soda (sodium bicarbonate) or potassium bicarbonate over the fire. The heat of the fire releases CO2 gas from the baking soda, which cuts off the fire.

When an LPG cylinder catches fire, it is wrapped with a wet cloth. It lowers the temperature of the fire and cuts off its contact with oxygen.

Types of Combustion

There are various types of combustion. Three important types of combustion are as follows.

Rapid Combustion

The combustion in which a large amount of heat and light are produced in a short time, is called rapid combustion. An Example:

Immediate burning of cooking gas (LPG) in a gas stove when a burning matchstick lighter is brought near it.Burning of kerosene oil in a kerosene stove, etc.

Spontaneous Combustion

The combustion in which a material suddenly bursts into flames without any external heat is called spontaneous combustion.

The heat required to start the spontaneous combustion is produced internally by the chemical reaction of the substance with oxygen. Example. White phosphorus burns in the air at room temperature on its own.

Some spontaneous combustion can be very dangerous. For example,

The spontaneous combustion of coal dust has resulted in many disastrous fires in coal mines.

Sometimes, due to the heat of the Sun or due to the spark of lightning from the sky, spontaneous combustion of straw (dry grass) and forest wood takes place, which leads to forest fire.

3. Explosions

A sudden combustion in which a large amount ofheat, light, and sound are produced is called an explosion. A large amount of gases formed is released quickly in an explosion, for Example.

When a cracker is ignited, an explosion takes place with the evolution ofheat, light, and sound. An explosion can also take place if pressure is applied to the cracker.

CBSE Class 8 Science Chapter 4 Flame

A flame is a region where combustion of a gaseous substance or vapour takes place.

The substances which vaporise during burning give flames, an Example. Kerosene oil and molten wax rise through the wick and are vaporised during burning and form flames.

While charcoal does not vaporise and so does not produce a flame, it just glows.

Given below is the list of some materials that either form or do not form a flame on burning.

Types Of Flame

Based on the availability of oxygen, a flame can be of the following types

1. When the supply of oxygen is not sufficient, the fuels burn incompletely, producing mainly a yellow flame. For example, the colour of a candle flame is yellow.
2. When the supply of oxygen is sufficient, the fuels burn completely, producing mainly a blue flame. For example, in LPG stoves, the LPG burns with a blue flame.

Structure of a Flame

A flame consists of three zones.

Innermost Zone: It is a dark or black coloured zone, which consists of hot, unburnt vapours of combustible material. It is the coolest part of the flame.

Middle Zone: It is the bright and luminous zone that produces a moderate temperature. Here, partial combustion of the fuel takes place.

Outermost Zone: It is a non-luminous zone. Here, the fuel’s complete combustion occurs because there is plenty of air around it. It has the highest temperature in the flame, i.e. is the hottest part of the flame.

This is the reason why goldsmiths blow the outermost zone of a flame with a metallic blow pipe for melting gold and silver to form the desired shapes for making jewellery.

CBSE Class 8 Science Chapter 4 Fuel

A substance which produces heat on burning is called a fuel. For example. Wood, charcoal, petrol, kerosene, LPG, natural gas, etc. These are the sources of heat energy. These fuels can be in solid form. Liquid or gaseous state.

A good fuel has the following characteristics

1. Readily available
2. Cheap
3. Burns easily in air at a moderate rate.
4. Produces a large amount of heat.
5. Does not leave behind any undesirable substances.
6. Low ignition temperature

Ideal Fuel

An ideal fuel is readily available, cheap and has a high calorific value and produces less amount of I pollution. There is probably no fuel that could be considered an ideal fuel.

CBSE Class 8 Science Chapter 4 Fuel Efficiency

Different fuels produce different amounts of heat on burning. The amount ofheat energy produced on complete combustion of 1 kg of a fuel is called its calorific value. The calorific value ofa fuel is expressed in a unit called kilojoules per kg (kJ or kg).

Calorific values of some fuels are given below

Burning of Fuels Leads to Harmful Products

The increasing fuel consumption has harmful effects on the environment because burning fuels produces harmful products that pollute the air around us.

These are as follows.

1. Hunting of carbon fuels like wool, coni nod petroleum rclc.tse unburnt carbon particles in the air, which are not a dangerous pollutant, are causing respiratory diseases such as asthma.
2. Also, the use of wood as a fuel leads to deforestation, which is quite harmful to living beings.
3. Incomplete combustion of carbon fuels gives carbon monoxide gas, which is a very poisonous gas. Excessive inhaling of this gas can kill a person.

It is dangerous to burn coal in a closed room. The carbon monoxide gas produced can kill a person sleeping in that room. So, we should never sleep in a room with a burning or smouldering coal fire in it.

1. Burning of most fuels releases carbon dioxide into the air. Increased concentration of carbon dioxide in the air is believed to be the cause of the rise in temperature of the atmosphere of Earth.
2. This is called global warming. Due to this, the ice in polar regions will melt very fast, producing a lot of water. It may cause a rise in the sea level, leading to floods in coastal areas.
3. Burning of coal and diesel releases sulphur dioxide gas. It is an extremely suffocating and corrosive gas. It may damage our lungs. Moreover, petrol and diesel engines give off gaseous oxides of nitrogen.

These oxides dissolve in rainwater and form acids. Such rain is called acid rain. It is very harmful for crops, buildings, and soil.

Compressed Natural Gas (CNG)

The use of diesel and petrol as fuels in automobiles is being replaced by CNG (Compressed Natural Gas) because it produces harmful products in very small amounts. CNG is a cleaner fuel.

Acid rain. When oxides of sulphur and nitrogen react with water molecules in the atmosphere, they form an acid. This acid mixes with rainwater and causes acid rain.

Calorific value is the amount of heat energy produced on the complete combustion of 1kg of fuel.

Combustion is a process in which a substance reacts with oxygen and gives off heat or light.

Deforestation, the Cutting of trees, leads to deforestation, which is quite harmful to the environment.

Explosion: When a cracker is ignited, a sudden reaction takes place. A large amount of gases is evolved in the process along with heat, light and sound. Such a reaction is called an explosion.

A flame is a region where combustion of gaseous substances or vapour takes place.

Fire extinguishers. These are used to block the contact of fuel with the promoter of combustion, such as oxygen.

Fuel is a combustible substance that catches fire easily and produces heat energy.

Fuel efficiency is the amount of heat that a fuel produces for a fixed amount of mass.

Global warming It is the process of rise in the average temperature of the Earth’s atmosphere due to the release of carbon dioxide on combustion of fuels.

Ideal fuel: An ideal fuel is cheap, readily available, readily combustible, and easy to transport.

Ignition temperature is the minimum temperature that must be reached before a substance starts burning.

Inflammable substances, these substances have a lower ignition temperature and burn easily with slight heating.

CBSE Class 8 Science Chapter 13 Light Makes Things Visible Notes

We can see objects when light emitted from them reaches our eyes. Those objects that emit light are known as luminous objects, e.g. Sun, a light bulb, etc.

However, some objects like a table or books do not have their own light; these are known as non-luminous objects. We can see these non-luminous objects if light rays get reflected from their surfaces and reach our eyes.

Read And Learn More CBSE Class 8 Science Notes

CBSE Class 8 Science: Simple experiments to show refraction of light, class 8 Reflection of Light

When a ray of light falls on any smooth surface or mirthen the direction of light changes. This phenomenon is known asthe  reflection of light.

Some important terms related to the reflection of light

• Incident ray The light ray that strikes the surface is knowthe the n as the incident ray. In the figure, AO is the incident.
• Reflected ray The light ray which comes back from the surface after reflection is known as reflected ray. In figure, OB is the reflected ray.
• Normal It is an imaginary line which is perpendicular to the mirror at the point of incidence. Inthe the  figure, OM is the normal ray.
• Angle of incidence The angle between incident ray and the normal is known as angle of incidence. In figure, ZAOM is the angle of incidence.
• Angle of reflection The angle between reflected ray and the normal is known as angle of reflection. In figure, ZBOM is the angle of reflection.

CBSE Class 8 Science Laws of Reflection

Reflection of light follows certain laws known as the laws of reflection. There are two laws of reflection as given below

1. First Law of Reflection: The angle of reflection is always equal to the angle of incidence (Zi = Zr).
2. Second Law of Reflection: The incident ray, the normal at the point of incidence, and the reflected ray all lie in the same plane

Science Insight The study of light and its behaviour while interacting with other objects is known as optics. Euclid studied the reflection and proposed the laws of reflection.

CBSE Class 8 Science Types Of Reflection

There are two types of reflection

• Regular reflection When a beam of parallel light rays is incident on a smooth plane surface, the reflected rays will also be parallel. This type of reflection is called regular reflection, e.g., Reflection from a plane mirror ray.

Since, plane mirror shows regular reflection, so we can see dear images of objects in a plane mirror.

• Diffused or irregular reflection When a beam of parallel light rays is incident on a rough surface, the reflected rays scatter in different direction. This type of reflection is called diffused or irregular reflection. e.g. Reflection from rough wall.
• The first law of reflection is also followed by irregular reflection. Irregular reflection forms a blurry image because reflected rays scatter in different directions.

Image Formation By A Plane Mirror

1. There are some characteristics of the image formed by a plane mirror
2. The image formed by a plane mirror is erect.
3. The image formed by the plane mirror is of the same size as the object.
4. The image formed by a plane mirror is at the same distance behind the mirror as the object is in front of it.
5. The image formed by a plane mirror is behind the mirror.
6. The image formed by plane mirrors is laterally inverted, i.,e. The left side comes to the right, and the right side comes to the left

CBSE Class 8 Science: Reflected Light Can Be Reflected Again

When a reflected light falls on a plane mirror, it gets reflected again. It also follows the law of reflection. Example:, In a barber’s shop, we see the back of the head using multiple reflections of two mirrors.

Similarly, a periscope workthe s on principle of the multiple reflection of two mirrors, where the beam of light from the object is reflected twice by the two mirror surfaces.

Multiple Images

We knowthat, a plane mirror forms only a single image of an o,,bject but when two plane mirrors are placed at some angle, the light rays reflected by the first mirror strike the second mirror and get reflected again. This results in the formation of multiple images.

This shows that the image of the object formed in the first plane mirror acts as an object for the second plane mirror.

Multiple Images Formed by a Plane

Mirror Placed at Different Angles

• When the angle between mirrors is more than 90°, then two images are formed.
• When the angle between mirrors is less thathe°, then several images increase.
• When an object is kept between two Parallel mirrors, then the reflected image is formed.

Note Number of images of an object formed when two plane mirrors are inclined at an angle is given by

⇒ \(\frac{360^{\circ}}{\theta(\text { given angle })}-1\)

Kaleidoscope

• A kaleidoscope is an optical instrument with two or more reflecting surfaces tilted to each other at an angle.
• It is used by designers, wallpaper and fabric artists to get ideas for new patterns.
• An interesting feature of a kaleidoscope is that you will never see the same pattern again.
• It is based on the principle of multiple reflections of light.

Construction

• It contains three plane mirrors inclined at an angle of 60° to each other in a tube.
• One end of tube is closed with two circular glass discs, the inner one is transparent glass and the outer one is ground glass.
• The other enthe the d of tube is closed by a cardboard disc aah the small hole in the centre.
• Glass pieces and bangle pieces of various colours are kept between the two discs.

Note: Ground glass is a type of glass that is partially transparent

CBSE Class 8 Science Sunlight: White or Coloured

The sunlight is considered as white light but actually it contains seven colours. These seven colours can be seen by passing sunlight through a prism.

When this sunlight or white light passes through the prism, it splits into seven different colours, and this process is called dispersion of light. One of the examplesampthe le of the dispersion of light is the n of rainbow.

CBSE Class 8 Science HumanTheThe eeeyee

The eye is a very important sense otthats used to see objects around us.

Structure Of The Eye

The shape of the eye is roughly spherical, It contains the following parts

• The outer coat of the eye is white. It is tough so that it can protect the interior parts of the eye from any accident.

• Cornea The transparent part of the outermost layer is cornea. The main function of cornea is to protect the eye but it also helps in focusing light due to its shape.
• Iris It is coloured part of eye situated just behind the cornea. The colour of iris can be brown, green or blue. It controls the amount of light entering in the eye.
• Pupi: The small black opening in the iris is a pupil. The size of the pupil is controlled by the iris.

Note: In dim light, our pupil expands to allow more light to enter our eyes, while in bright light, it contracts.:

• Lens: Just behind the pupil, a transparent and elastic structure is present. It is thicker in the cent,re just like a convex lens. It focuses the light on the retina.
• Ciliary muscles The lens is attached to ciliary muscles. These muscles are used to change the shape of eye lens to focus image on the retina.
• Retina A screen on which the image is formed in the eye is known as retina. The retina contains many optic nerves. These optic nerves carry image in the form of electrical signal to the brain.

Optic nerves. There are two types of cells in the optic nerve

1. Cones.. These are sensitive to bright light. It is responsible for colour vision.
2. Rods. These are sensitive to dim light. It is responsible for vision in dim light.

Blind spot At the junction ofoptic nerve and the retina , there are no sensory cells. The brain cannot sense image formed at this spot. Hence, it is named as blind spot.

Eyeli.The part of an eye that prevents any object from entering the eye is known as the lids. It shuts out if light is not required.

Persistence Of Vision

Any image formed on the retina does not vanish immediately. It persists there about \(\frac{1}{16}\) th of a second. Due to this fact, if steady images of a moving object are flashed on the eye at a rate faster than \(\frac{1}{16}\) then the eye perceives this object as moving. The same phenomenon is used in movies.

CBSE Class 8 Science Range and Defect of Vision

• The minimum distance at which the normal eye can see objects is 25 cm.
• Some people can see objects close to them cl,early but cannot see distant objects (short-sightedness). On the other hand, some can see distant objects clearly’but are unable to see nearby objects (long-sightedness).

These eye defects can be corrected by using the proper power ofthe the lens.

• In old age,e eyesightbecomes foggy due to a cloudy eye lens, which is known as cataract. This defect can be corrected by getting proper surgery.

Science Insight Short-sightedness is known as myopia, whereas long-sightedness is known as hypermetropia.

Care Of Eyes

To avoid the above defects, take care of your eyes steps below.

1. Do not rub your eyes. If dust particles go into your eyes, then wash it gently. If there is no improvement, then go to a doctor.
2. Do not look at a powerful source of light like the sun or a laser.
3. Insufficient light can cause eye strain, headaches, or injury to the retina.
4. Always read at the normal distance of vision.
5. If advised by your specialist, then use spectacles.
6. Take a proper, balanced diet in your meals, especially vitamin A, which cures the problem of night blindness

CBSE Class 8 Science Eyes Of Other Animals

Different animals have different eye shapes; some of them are given as follows.

1. Crab Its eyes are very small and presethe nt on sstalks so the crab cannot see its enemy even if it approaches from behind.
2. Butterf.ly They have large compound eyes made up of thousands of little eyes. They can see in all directions. It can sense even smovementsement in an object.
3. Owl Its eyes have a large cornea and pupil that allows more light to enter the eye. Also, it has few cones and a large number of rods. Due to this, the owl can see in the night.
4. Day birds have a large number of cones, which are sensitive to bright light.

Visually Impaired Persons can Read and.. Write

Some people cannot see due to any eye defect but they can identify things using other senses like touching and listening.

Based on these senses, some optical aids and non-optical aids are used to improve the life of a visually impaired person.

1. Non-optical aid: It includes the following. aids
2. Visual AI. These aids can magnify words and provide the ideal intensity of light and material at proper distances.
3. Tactual AI. These aids are based on a sense of touch.h, Example. Braille wr, iter state, and stylus.
4. AuditoryAIi.These are based on hearing. Example: Cassettes, talking books, etc.
5. Electronic.Aid.ss These aids are based on electronic devices, for  Example. Talking calculators and computers.

• Optical: aids are arc optical devices like bifocal lenses, contact lenses, telescopic aids, etc.

Braille System

Braille is a written language for visually challenged person. This system was adopted in 1932. It has series of patterns of raised dots. These dots represent specific alphabets and numbers. There arc 63 dot patterns. Combination of these patterns produces words and grammatical sign. Visually challenged people can recognise these words by touching them.

• Laws of reflection: These are sets of two universal laws that are followed by any light rays during reflection.
• Angle of Incidence: The angle betweenween incident and noise is now as the angle of Incidence.
• Angle of reflection The angle between ween reflected ray and the normal is known as the angle of reflection.
• Cornea: It is the transparent front part which helps in focusing light on the t onTheeiris The colored part of the eye is situated just behind the cornea.
• Pupil: The small opening in the middle of the iris through which light ens in the eye.:
• Lens: A transparent and elastic structure present behind the pupil. It is thicker in the centre.
• Rods: These cells are present in the retina and are sensitive to dim light.
• Cones: These cells are sensitive to bright light.
• Retina: It is a part of the eye that acts as a screen.
• Cataract: It is a defect of thein whichwhicthe eye lens becomes cloudy and eyesight becomes hazy.
• Braille is a written language for visually challenged people.

CBSE Class 8 Science Chapter 2 Microorganisms: Friend And Foe Detailed Notes Groups Of Microorganisms

The microorganisms may be unicellular (single-celled) or multicellular (many-celled). These are classified into four major groups as described below

Bacteria are the simplest unicellular organisms with a rigid cell wall, like plant cells, an Example. Lactobacillus, E. coli, etc.

Fungi are a large group of organisms which do not have chlorophyll and hence, do not photosynthesise. Thus, they are heterotrophs, an Example. yeast, Penicillium, Aspergillus, Rhizopus (bread mould). Fungi may be unicellular or multicellular.

Protozoa is a group of single-celled microorganisms that have animal-like characteristics. They can move from place to place. Example, Amoeba and Paramecium. Diseases like dysentery and malaria are caused by protozoa.

Algae is a large group of single-celled, plant-like organisms. They have no true roots, stems or leaves. They can be unicellular or multicellular. Algae are photoautotrophic organisms that carry out photosynthesis. Example: Chlamydomonas, Spirogyra, etc.

Viruses

These are also microscopic organisms, but are different from other microorganisms. They reproduce only inside the cells of the host organisms. These host organisms can be a bacterium, plant or animal.

Class 8 Science Chapter 2 Microorganisms: Friend And Foe Notes: Where Do Microorganisms Live?

Microorganisms spread over the entire biosphere. Examples in soil, water, air, and inside the bodies of animals, including humans, etc. They are characterised by a very high degree of adaptability, as they can survive in all types of environments, ranging from ice-cold climates to hot springs and deserts to marshy lands.

Some microorganisms grow on other organisms, while others exist freely. Microorganisms like amoebas can live alone, while bacteria and fungi may live in colonies.

Microorganisms And Us

Microorganisms play a dual role in our lives. They not only offer various benefits, but also cause a significant number of diseases.

Friendly Microorganisms

Microbes are used for various purposes. Some common beneficial activities of microbes are discussed below

1. Microbes in Household Products

Some of the household products obtained from microbial activities are as follows

Curd contains several microorganisms. Of these, the bacterium Lactobacillus promotes the formation of curd. It multiplies in milk and converts milk into curd, The curd is added to run idlis and bhaturas to make them soft and spongy.

Baking Industry The dough for making bread, cakes and pastries is prepared by adding yeast to flour.

Yeast reproduces rapidly and produces carbon dioxide (CO2) during respiration; bubbles of this gas fill the dough and increase its volume. It adds sponginess to cake and bread.

Bacteria are also involved in the making of cheese, pickles and other food items. Bacteria, along with yeast, are helpful for the fermentation of rice idlis and dosa batter.

2. Commercial Use of Microorganisms

Microbes are used for the large-scale production of alcohols, wine and acetic acid (vinegar). Yeast is used for the commercial production of alcohol and wine. For this purpos,e yeast is grown on natural sugars present in grains like barley, wheat, rice, crushed fruit juices, etc.

The process of converting sugar into alcohol is called fermentation. Louis Pasteur discovered fermentation in 1857.

3. Medicinal Use of Microorganisms

When an individual falls ill, the doctors prescribe antibiotics for treatment. The source of these antibiotic medicines and vaccines is microorganisms. The details of antibiotic medicines and vaccines are given below

Antibiotics: The medicines, which are used to kill or stop the growth of the disease-causing microorganisms, are known as antibiotics.

These antibiotics in the form of tablets, capsules or injections are products of microorganisms (mainly bacteria and fungi), For example. penicillin, streptomycin, tetracycline, erythromycin.

The antibiotics are manufactured by growing specific microorganisms and are used to cure a variety of diseases. Antibiotics are even mixed with the feed of livestock and poultry to check microbial infection in animals.

They are also used to control many plant diseases. Antibiotics are not effective against cold and flu as these are caused by viruses.

Certain precautions to be taken for using antibiotics are as follows

It should be taken only on the advice of a qualified doctor in the proper dose. In wrong doses (or taken when not needed), it becomes less effective when taken in future

The person must complete the course as prescribed by their doctor.

It should not be taken unnecessarily, as it may kill useful bacteria in the body.

History Of Antibiotics: Penicillin

The first antibiotic, penicillin, was discovered by Alexander Fleming in 1929. He was working on a culture of disease-causing bacteria.

During the experiment, he observed that the presence of a mould in one of his culture plates prevented the growth of bacteria. Indeed, it also killed many of these bacteria. From this mould, penicillin was prepared.

Vaccine: It is a biological preparation that provides immunity (or protection) against a particular disease.

A vaccine consists of dead or weakened microbes which, when introduced in a healthy body, produces suitable antibodies to fight and kill the invading microbes. bacteria or microbes.

These antibodies remain in the body to protect from disease-causing microbes, thus developing immunity. The diseases which can be prevented by vaccination are polio, cholera, tetanus, rabies, TB, smallpox, measles and hepatitis.

Vaccination

• Edward Jenner discovered the smallpox vaccine in 1798.
• A worldwide campaign against smallpox has finally led to its eradication from most parts of the world.
• Pulse Polio Immunisation Programme was launched in India in 1995 for the eradication of polio. Children in the age group of 0 (zero) months to 5 years are administered polio drops (actually vaccine) every year.

4. Increasing Soil Fertility

Some bacteria and cyanobacteria (blue-green algae) can fix nitrogen from the atmosphere to enrich soil with nitrogen and increase its fertility.

These microbes are commonly called biological nitrogen fixers, e.g. Rhizobium.

5. Cleaning the Environment

Microorganisms decompose dead organic waste of plants and animals, converting them into simple substances. These substances are again used by other plants and animals. The microorganisms are used to degrade the waste and smelly substances from our biosphere and thereby clean the environment.

Harmful Microorganisms

The microorganisms are harmful in many ways. Some of these cause diseases in human beings, plants and animals. Such disease-causing microorganisms like bacteria, fungi, viruses and protozoans are called pathogens.

They enter the body by various means, where they multiply and cause harm to the body. Some of their activities are described below.

Disease-Causing Microorganisms in Humans

Certain microbial diseases are spread from an infected person to a healthy individual through air, water, food or physical contact. Such diseases are called communicable diseases, for Example. cholera, common cold, chickenpox and tuberculosis.

Transmission of Communicable Diseases

There are various ways through which communicable diseases can be transmitted. These are as follows

1. When a person suffering from a common cold or flu sneezes, coughs or spits, fine droplets of moisture carrying viruses spread in the air. The virus may enter the body of a healthy person while breathing and cause infections, for Example tuberculosis.
2. Through direct contact with an infected person, for Example, chickenpox, ringworm.
3. Through the food we eat or the water we drink, for Example, cholera, typhoid.
4. Through vectors or carriers, like some insects and animals. The organisms that transmit pathogens from an infected individual to a healthy one are known as carriers. For example,

The housefly is one such carrier. The flies sit on the garbage and animal excreta, accumulating pathogens on their bodies. When these flies land on exposed food, they can transfer these pathogens.

Another example of a carrier is the female Anopheles mosquito, which carries the parasite of malaria (Plasmodium). The female Aedes mosquito acts as a carrier of the dengue virus.

Prevention Of Communicable Diseases

Some methods of preventing the Occurrence and spread of communicable diseases are as follows

1. Cover your mouth and nose with a handkerchief while sneezing.
2. Keep your food covered and avoid consuming uncovered food items.
3. Drink clean and safe water.
4. Use mosquito nets and mosquito repellent creams to protect yourself from mosquito bites.
5. Take the vaccination at the proper time.
6. Avoid breeding of mosquitoes by not allowing water to collect anywhere, i.e. in coolers, tyres, etc. and by keeping surroundings clean and dry.

Some of the common diseases affecting humans, their mode of transmission and a few general methods of prevention are given in the table below.

Common human diseases caused by microorganisms

Disease-Causing Microorganisms In Animals

Several microorganisms not only cause diseases in humans, but also cause diseases in other animals. For example, Anthrax is a dangerous human and cattle disease caused by a bacterium. Some of the common animal diseases caused by microorganisms are given in the table below

Common Animal Diseases Caused by Microorganisms

Disease-Causing Microorganisms In Plants

Several microorganisms cause diseases in plants like wheat, rice, potato, sugarcane, orange, apple, etc.

These diseases reduce the yield of crops. They can be controlled by the use of certain chemicals which kill the microbes. Some of the common plant diseases caused by microorganisms are tabulated below.

Common plant diseases caused by microorganisms

Food Poisoning

Microorganisms that grow on food may produce toxic substances, thereby causing the spoilage of food.

The spoiled food starts giving a foul smell, bad taste, and its colour may also change “his spoiling of food is a chemical change. Consumption of spoiled food can lead to a condition called food poisoning.

The symptoms of food poisoning include vomiting, diarrhoea, headache, fever, etc. It can cause serious illness and even death. So we must preserve food to prevent it from getting spoiled.

Food Preservation

The process by which the spoilage of food is prevented using suitable chemical or physical methods is called food preservation. Some of these methods are given below

Chemical method. The chemicals used to check the growth of microorganisms are called preservatives. Salts, edible oils and acid preservatives are added to pickles to prevent the attack of microbes.

Sodium benzoate and sodium metabisulfite are common preservatives used to check the growth of microbes. They are added in jams and squashes to check their spoilage.

Preservation by common salt has been used to preserve meat and fish for ages. Salting is also used to preserve amla, raw mangoes, tamarind, etc.

Preservation by sugar: Jams, jellies and squashes are preserved by sugar. It reduces the moisture content, which inhibits the growth of bacteria which spoil food.

Preservation by oil and vinegar. Use of oil and vinegar prevents the spoilage of pickles because bacteria cannot live in such an environment. Vegetables, fruits, fish and meat are preserved by this method.

Heat and cold treatments. Boiling kills many microorganisms. Similarly, we keep our food in the refrigerator as low temperature inhibits the growth of microbes.

Pasteurisation is used for the preservation of milk. It involves the process of heating milk to about 70%  for 15 to 30 seconds (killing most of the bacteria) and then cooling it quickly at a very low temperature.

This milk is then stored at a cold temperature. Pasteurised milk can be consumed without boiling as it is free from harmful microbes. The milk that comes in packets does not get spoiled as it is pasteurised. The process of pasteurisation was discovered by Louis Pasteur.

Storage and packing. These days, dry fruits and even vegetables are sold in sealed, air-tight packets. This helps to preserve food by keeping germs and microbes out. Airtight packets also prevent food from moisture and contamination.

Science Insight: Irradiation is a modern method of food preservation. The food is exposed to high-energy gamma rays or X-rays. These powerful radiations destroy the harmful microorganisms. The food can be preserved for years without refrigeration.

Nitrogen Fixation

Plants cannot directly utilise the nitrogen gas present in the atmosphere. It is taken up by plants from the soil only when it is converted to nitrogenous compounds. The process of conversion of atmospheric nitrogen into nitrogenous compounds is called nitrogen fixation.

Nitrogen can be fixed by

1. Lightning is a natural process that converts nitrogen into a usable form.
2. Rhizobium bacteria are involved in the fixation of nitrogen in leguminous plants. These bacteria live in the root nodules of leguminous plants such as beans and peas, with which it has a symbiotic relationship.
3. Some of the blue-green algae can fix nitrogen, e.g. Anabaena, Nostoc, etc.

Although plants and animals continuously use up atmospheric nitrogen, it remains constant. This happens through a cycle of processes called the nitrogen cycle.

Class 8 Science Chapter 2 Microorganisms, Friend And Foe Notes With Practice Questions Nitrogen Cycle

It is a repeating cycle of processes during which nitrogen in the atmosphere circulates through living and non-living things. Nitrogen is one of the essential constituents of all living organisms.

It is a part of proteins, chlorophyll, nucleic acids and vitamins. Our atmosphere contains 78% nitrogen gas. The main steps in this cycle are as follows

1. The atmospheric nitrogen is fixed through various means and is converted into usable compounds.
2. These compounds are utilised by plants from the soil through their root system. This nitrogen is used for the synthesis of plant proteins and other compounds.
3. Animals taking these plants as food get these proteins and other nitrogen compounds.
4. When these plants and animals die, certain bacteria and fungi convert the complex nitrogenous compounds present in their bodies into simple nitrogenous compounds. These simple nitrogenous compounds are again absorbed by the plants for their growth.
5. Certain other bacteria convert some of these compounds into nitrogen gas, which goes back into the atmosphere. As a result, the percentage of nitrogen in the atmosphere remains nearly constant.

Algae are A type of microorganism that can photosynthesise and form green patches on water.

Antibiotics are the medicines that kill or stop the growth of disease-causing microorganisms.

Antibodies are substances produced by our body to fight against the invading microorganisms.

Bacteria are A type of microorganism which can be found in all kinds of environments.

Carrier Animals that carry disease-causing microorganisms.

Communicable diseases are Microbial diseases that can spread from an infected person to a healthy person through air, water, food or physical contact.

Fungi are A type of microorganism which cannot photosynthesise.

Fermentation is A process of conversion of sugar into alcohol by bacteria and yeast.

Lactobacillus is A type of bacteria that promotes the formation of curd.

Microorganism: A living organism which cannot be seen with the unaided eye.

Nitrogen fixation is the Process of conversion of free nitrogen gas present in the atmosphere into nitrogenous compounds.

Nitrogen cycle: The nitrogen in the atmosphere is circulated again and again in a cyclic manner by the living and the non-living things. This circulation is called the nitrogen cycle.

Protozoa, A type of microorganism that has animal-like characteristics.

Pathogen, a disease-causing microorganism.

Preservatives are the chemical substances used to prevent the growth of microorganisms.

Pasteurisation Process of destroying microorganisms in food items, e.g. milk. It involves high heating followed by sudden cooling.

Rhizobium, A Bacteria that live in the roots of leguminous plants and is involved in the fixation of atmospheric nitrogen.

Virus: A type of microorganism which can reproduce only inside a host.

Vaccine: Weak or dead microbes that are injected into our body to fight against disease.

Yeast is A unicellular fungus that is used in the baking industry.

Human Eye And Colourful World Short Answer Questions

Question 1. A star appears slightly higher than its actual position in the sky. Explain it.
Answer: It is due to atmospheric refraction. When the light of a star enters into the atmosphere, it is refracted continuously. The higher level of air acts as a rarer medium while the dense air near the surface of the earth acts as a denser medium. So, the atmosphere bends the starlight towards the normal.

As a result, the apparent position of stars is slightly different from its actual position. Thus the star appears slightly higher than its actual position in the sky.

Question 2. When we place a glass prism in the path of a narrow beam of white light, a spectrum is obtained. What happens when a second identical prism is placed in an inverted position to the first prism? Draw a labelled ray diagram to illustrate it.

The second prism recombines the spectrum obtained by the first prism. So from the other side of the second prism, n beam of white light is again obtained.

Question 3. With the help of scattering of light, explain the reason for the difference in colours ofthe sun as it appears during sunrise/sunset and noon.
Answer: The sun appears reddish at sunrise as well as at sunset and appears white at noon when it is overhead.

At sunrise/sunset, the blue light of shorter wavelength gets scattered away while passing through the thicker layer of air/large distance through air and red light of longer wavelength reaches our eye.

At noon, the sun is overhead and the light of the sun travels a relatively shorter distance to our eyes little blue and violet colours are scattered, and white light reaches our eyes.

Question 4. Why do some people use bifocal lenses?
Answer: Some people suffer from both myopia and hypermetropia. Such people require bi-focal lenses. The upper part ofthe bi-focal lens is a concave lens used for distant vision while its lower part is a convex lens used for reading purposes. These days, refractive defects are also corrected by using contact lenses or through surgical interventions.

Question 5. What is the “power of accommodation ofthe eye”? What happens to the image distance when the object being viewed is moved away from the eye?
Answer: The power of accommodation of the eye is the ability of the eye lens to adjust its focal length to see nearby and distant objects.

Image distance in the eye is fixed, being equal to the distance of the retina from the eye lens.

Question 6. Why do we observe the seven colours when white light passes through a glass prism? Which component of white light deviates the least?
Answer: When light passes through a prism offers different refractive indices to the different wavelengths of light. Thus, each colour suffers a different deviation. The red light deviates the least.

Question 7. Calculate the maximum power of accommodation of a person having normal vision.

A person needs to use glasses to read a newspaper. Identify the defect in her vision and the type oflens she would need to correct it.

Sometimes when we enter into a dark room from bright sunlight we are unable to see objects. Why?

Answer: D = 25 cm, Now, P = 1/f- D = 100/D = 100/25 = 4 D, thus for a person having normal vision, the power of accommodation is about 4 dioptre.

Long-sightedness, a convex lens.

The pupil regulates and controls the amount of light entering the eye. In bright sunlight, the size ofthe pupil is small and when we enter a dark room it takes some time for the pupil to expand in size due to dim light.

Question 8. A person can see up to 3 metres only. Prescribe a lens for him so that he can see up to 12 metres.
Answer: Clearly, the lens used in this case should be such that the rays proceeding from a point distant 12 m from the eye should, after refraction, appear to come from a point distant 3 m from the eye. Here, u =- 12 m, v =- 3 m, /= ?

Using lens formula, \(\frac{1}{f}=\frac{1}{v}-\frac{1}{u}\)

⇒ \(\begin{aligned}
& =-\frac{1}{3}+\frac{1}{12} \\
& =-\frac{1}{4} \quad \text { or } f=-4 \mathrm{~m}
\end{aligned}\)

Thus, a concave lens of a focal length of 4 m should be used.

Question 9. A narrow beam PQ of white light is passing through a glass prism ABC as shown in the diagram.

1. Trace it on your answer sheet and show the path of the emergent beam as observed on the screen DE.
2. Write the name and cause ofthe phenomenon observed.
3. Where else in nature is this phenomenon observed?
4. Based on this observation, state the conclusion which can be drawn about the constituents of white light.

Answer: The diagram is shown below

The phenomenon is called the dispersion of light. It is because ofthe reason that different wavelengths oflight travel at different speeds in the glass prism. In a rainbow. White light consists of seven different wavelengths viz. violet, indigo, blue, green, yellow, orange and red.

Question 10. A person with a myopic eye cannot see objects beyond a distance of 1.5 m. What would be the power ofthe corrective lens used to restore proper vision?
Answer: The remedial lens should make the object at infinity appear at the far point.

1. For an object at infinity, u =-∞
2. Far point distance of the defective eye, v =- 150 cm
3. By lens formula,

⇒ \(\begin{aligned}
\frac{1}{f} & =\frac{1}{v}-\frac{1}{u} \\
& =\frac{1}{-150}-\frac{1}{-\infty} \\
& =-\frac{1}{150}+0
\end{aligned}\)

⇒ \(=-\frac{1}{150}\)

or, f=-150cm.

Power, \(\mathrm{P}=\frac{1}{f(\text { in } \mathrm{m})}\)

A negative sign shows that the remedial lens is concave.

Question 11. Why do we have two eyes for vision and not just one?
Answer: Advantage of binocular vision: There are many advantages of having two eyes instead of one. These are as follows.

1. It gives a wider horizontal field of view of about 180°.
2. It helps to detect even fainter objects.
3. It provides a three-dimensional effect of objects around us.
4. The two eyes give relief to each other after every fraction of a second.

Question 12. A person is unable to read a book clearly when kept at a distance of 25 cm from his eye. Name the defect. How can it be corrected? Draw ray diagrams for defective eye-corrected eyes and explain them.
Answer: Hypermetropia or long-sightedness.

It is corrected by using a convex lens of suitable focal length. The diagrams are as shown below:

Defective eye: Rays from the object at N are focused beyond the retina i.e., near point goes away from the eye.

Corrected eye: Using a converging lens makes the image at the retina.

Question 13. Explain why the colour of the clear sky is blue.
Answer: The molecules of air and other fine particles in the atmosphere have a size smaller than the wavelength of visible light.

These are more effective in scattering light of shorter wavelength at the blue end than light of longer wavelength at the red end. The red light has a wavelength about 1.8 times greater than that of blue light.

Thus, when sunlight passes through the atmosphere, the fine particles in the air scatter the blue colour (shorter wavelength) more strongly than red. The scattered blue light enters our eyes. If the earth had no atmosphere, there would not have been any scattering.

Question 14. State the cause of dispersion, when white light enters a glass prism. Explain with a diagram.
Answer: Light is made up of different colours. Each colour travels at its speed inside a prism. Due to this, different light colours bend through different angles for the incident ray, as they pass through a prism. The red light bends the least while the violet the most.

Thus, the rays of each colour emerge along different paths and thus become distinct. It is the band of distinct colours that we see in a spectrum. The diagram is as shown.

A beam of white light falling on a glass prism gets split up into seven colours marked 7 as shown in the diagram:

The colours at positions marked 3 and 5 are similar to the colour ofthe sky and the colour of gold metal respectively. Is the above statement made by the Is student correct or incorrect Justify?

Which of the above-shown positions corresponds approximately to the colour of:

1. A Brinjal
2. Danger Signal
3. Neel Which Is Applied To The Clothes
4. Orange

Answer: As a result of dispersion produced by the prism, the white light is split up into seven colours. These colours as we know are represented by the letters of the word ‘VIBGYOR’.

Thus, blue is in the 3rd position and yellow is in the 5th position. These are the colours of the sky and the gold medal respectively. Thus, the statement is correct.

The positions of the colours ofthe given objects are represented by:

1. brinjal ……….7
2. Danger singal…….1
3. Neel ……….6
4. Organe……2

Question 15. A person wears glasses of power -2.5 D. Is the person far-sighted or near-sighted? What is the point of the person without glasses?
Answer: P =- 2.5 D.

Negative power shows that the lens is concave, so the person is near-sighted.

⇒ \(\begin{aligned}
f & =\frac{1}{\mathrm{P}} \\
& =\frac{1}{-2.5} \mathrm{~m}=-\frac{2}{5} \mathrm{~m} \\
& =-40 \mathrm{~cm} \\
u & =-\infty, v=?
\end{aligned}\)

From the lens formula,

⇒ \(\frac{1}{v}=\frac{1}{f}+\frac{1}{u}=\frac{1}{-40}+\frac{1}{-\infty}=\frac{1}{-40}-0=-\frac{1}{40}\)

v=-40 cm

Thus, the far point of the eye is 40 cm from the eye.

Question 16. List two causes leading to myopia of the eye. Draw ray diagrams to show the image formation in case of defective eye and correct eye.
Answer: The two main causes of myopia are:

1. The eyeball is longer than normal.
2. Decrease in the focal length of the eye lens

Question 17. A person needs a lens of power -2.5 dioptres for correcting his vision. Name the defect of vision he is suffering from. Which lens will he be using for the correction1? Also, find the focal length oflens.
Answer: The person is using a concave lens of power -2.5 D. The person is, therefore, suffering from myopia. He is using a concave lens for the correction of the eye. Since P =1If, therefore, f= 1/P = 1/2.5 = 0.4 m = 40 cm.

Question 18. Explain how a normal human eye can see distinctly the object placed at a distance as well as those placed at a nearer distance. What are the far point and near point of a normal human eye?
Answer: For seeing distant objects, the focal length ofthe eye lens should be large and for seeing near objects the focal length of the eye lens should be small.

The focal length of the eye lens can be adjusted with the help of ciliary muscles holding the lens in position. The far point and near point are at infinity and 25 cm respectively.

Question 19. Name the part of the eye where the image is formed by the eye lens. What is the nature ofthe image formed? How is this image sent to the brain?
Answer: The image of the object formed by the eye lens is at the retina of the eye. The image formed on the retina is real and inverted. The image is sent to the brain with the help of the optic nerve.

Question 20. An old person is unable to see nearby objects as well as distant objects, What defect of vision is he suffering from? What kind oflens will be required to see the nearby as well as distant objects? Give reasons.
Answer: Presbyopia.

He shall have to use both kinds of lenses, convex lens for long-sightedness and concave lens for short-sightedness.

Question 21. Why does the sun appear reddish early in the morning? Illustrate with the help of a labelled diagram.
Answer: It is due to the scattering of light. The light near the horizon passes thicker layers of air and a larger distance of the atmosphere; hence, most ofthe blue light is scattered away and longer wavelength reaches our eyes giving rise to the reddish colour of the sun.

Question 22. Why does it take some time to see the objects in a dim room when we enter the room from the bright sunlight outside?
Answer: In bright light, the size ofthe pupil is small to control the amount of light entering the eye. When we enter a dim room, it takes some time so that the pupil to expand and allow more light to enter and help to see things.

Question 23. Why does it take some time to see objects in a cinema hall when we just enter the hall from, bright sunlight? Explain in brief
Answer: The pupil regulates and controls the amount oflight entering the eye. In bright sunlight, the size of the pupil is small and when we enter the cinema hall takes some time for the pupil to expand in size due to dim light.

Question 24. Describe the formation of the rainbow in the sky.
Answer: A rainbow is always formed in a direction opposite to that of the sun. The water droplets act like small prisms. They refract and disperse the incident sunlight, then reflect it internally, and finally refract it again when it comes out of the raindrop. Due to the dispersion oflight and internal deflection, different colours reach the observer’s eye.

1. What is meant by the least distance of distinct vision?
2. How does the thickness ofthe eye lens change when we shift looking from a distant tree
   to reading a book?

Answer: It is the minimum distance, to which normal eyes can see two similar objects distinctly.

The eye lens is comparatively thicker while reading a book.

Question 25. To correct distant vision, a person needs a lens of power -5.5 dioptres, and to correct near vision, he needs a lens of power +1.5 dioptres. What is the focal length of the lens required for correcting distant vision and near vision?

Answer: Power of distance viewing part of the lens,

P₁ =-5.5 D

The focal length of this part

⇒ \(f_1=\frac{1}{P_1}=\frac{1}{-5.5} \mathrm{~m}\)

= -18.73 cm.

As the power of the near-vision part is measured relative to the main part of the lens of power – 5.5 D, so we use

P₁ + P₂ = P

or — 5.5 + P₂ = + 1.5

or P₂ = + 6.5 D

Focal length near-vision part

⇒ \(\begin{aligned}
f_2 & =\frac{1}{\mathrm{P}_2}=\frac{1}{+6.5} \mathrm{~m} \\
& =+15.4 \mathrm{~cm}
\end{aligned}\)

Question 26. The far point of a myopic person is 80 cm in front of the eye. What is the nature and power ofthe lens required to correct the problem1
Answer: The remedial lens should make the objects at infinity appear at the far point.

∴ For an object at infinity u =-∞

Far point distance of the defective eye, v =- 80 cm

By lens formula, \(\frac{1}{f}=\frac{1}{v}-\frac{1}{u}=\frac{1}{-80}-\frac{1}{-\infty}=-\frac{1}{80}+0=-\frac{1}{80}\)

or f=-80cm

Power, \(\mathrm{P}=\frac{100}{f(\mathrm{in} \mathrm{cm})}\)

⇒ \(\begin{aligned}
& P=\frac{100}{-80} \\
& P=-1.25 D
\end{aligned}\)

A negative sign shows that the remedial lens is concave.

Question 27. Make a diagram to show how hypermetropia is corrected. The near point of a hypermetropic eye is 1 m. What is the power of the lens required to correct this defect? Assume that the near point ofthe normal eye is 25 cm.
Answer: The object placed at 25 cm from the correcting lens must produce a virtual image at 1 m or 100 cm.

u = -25 cm, v =- 100 cm

By lens formula \(\frac{1}{f}=\frac{1}{v}-\frac{1}{u}=\frac{1}{-100}-\frac{1}{-25}=-\frac{1}{100}+\frac{1}{25}=+\frac{3}{100}\)

or \(f=+\frac{100}{3} \mathrm{~cm}=+\frac{1}{3} \mathrm{~m}\)

Power, \(\mathrm{P}=\frac{1}{f}=+\frac{3}{1}=+3 \mathrm{D}\)

Question 28. Why is a normal eye not able to see the objects placed closer than 25 cm?
Answer: At a distance of less than 25 cm, the ciliary muscles cannot bulge the eye lens anymore, the object cannot be focussed on the retina and it appears blurred to the eye, as shown in the figure.

Object O within 25 cm from the eye is not focused on the retina and is seen blurred.

Question 29. What happens to the image distance in the eye when we increase the distance of an object from the eye?
Answer: Due to its property of accommodation, the eye lens of a normal eye forms images of objects at various distances on the same retina. So the image distance in the eye remains the same.

Question 30. Why do stars twinkle?
Answer: The apparent position of a star is slightly different from the actual position due to the refraction of starlight by the atmosphere. Due to the variation in the atmospheric conditions, the amount oflight from a particular star changes randomly with time. This gives rise to a twinkling effect of the star.

Question 31. Explain why the planets do not twinkle.
Answer: As the planets are much closer to the Earth, the amount oflight received from them is much greater and the fluctuations caused in the amount of light due to atmospheric refraction are negligible as compared to the amount oflight received from them.

Question 32. Why does the sun appear reddish early in the morning?
Answer: Early in the morning, the sun is near the horizon. Sunlight reaches us after covering a large thickness ofthe atmosphere.

So shorter waves of blue region are almost completely scattered away by the air molecules. Red waves of longer wavelength are least scattered and reach our eyes. The sun appears red.

Question 33. Why does the sky appear dark instead of blue to an astronaut?
Answer: The atmosphere is quite thin at very high altitudes. There is almost no scattering of sunlight. So the sky appears dark to an astronaut.

Question 34. What is meant by the power of accommodation of the eye?
Answer: The power of accommodation of the eye is the maximum variation of its power for focusing on near and far (distant) objects. for a normal eye, the power of accommodation is about four diopters.

Question 35. A student sitting at the back of the classroom cannot read the letters written on the blackboard. What advice will a doctor give to her? Draw a ray diagram for the correction ofthis defect.
Answer: The student is suffering from myopia or near-sightedness. In myopia, one can see nearby objects clearly but cannot see distant objects distinctly. The doctor advises a concave lens of suitable power to bring the image back into the retina.

The ray diagram for the correction of this defect is shown below

Question 36. A person needs a lens of power -4.5D for correction of her vision.

1. What kind of defect in vision is she suffering from?
2. What is the focal length ofthe corrective lens?
3. What is the nature ofthe corrective lens?

Answer: As the power of the lens is negative, she must be suffering from myopia.

Power, P =- 4.50, focal length, f=?

⇒ \(\mathrm{P}=\frac{1}{f} \Rightarrow f=\frac{1}{\mathrm{P}}=\frac{1}{-4.5}=-0.222 \mathrm{~m}=-22.2 \mathrm{~cm}\)

The nature ofthe corrective lens is concave or divergent.

Question 37. How will you use two identical prisms so that a narrow beam of white light incident on one prism emerges out ofthe second prism as white light? Draw the diagram.
Answer: A narrow beam of white light incident on one prism emerges out of the identical prism placed in an inverted position to the first prism close to the first prism as shown in the figure given below.

Question 38. Draw a ray diagram showing the dispersion through a prism when a narrow beam of white light is incident on one of its refracting surfaces. Also, indicate the order of the colours ofthe spectrum obtained.
Answer:

Question 39. Why do we see a rainbow in the sky only after rainfall?
Answer: We see a rainbow in the sky only after rainfall due to the dispersion of sunlight by tiny water droplets, present in the atmosphere due to rainfall. The water droplets act as a prism.

They refract and disperse the incident sunlight then reflect it internally and finally refract it again when it comes out of a raindrop hence due to the dispersion of light and internal reflection, rainbow formation takes place during rainfall.

Question 40. Why is the colour ofthe clear sky blue?
Answer: The blue colour of the clear sky is due to Rayleigh’s scattering of sunlight. The molecules of air and other fine particles in the atmosphere have sizes smaller than the wavelength of visible light.

These are more effective in scattering light of shorter wavelengths at the blue end than light of longer wavelengths at the red end.

Question 41. What is the difference in colours of the sun observed during sunrise-sunset and noon? Explain each.
Answer: During sunrise/sunset, the sun looks reddish because, at this stage, rays from the sun have to travel a much larger part of the atmosphere. The red colour having the largest wavelength is scattered the least.

At noon, the sun is nearly overhead. The sunlight has to pass through a smaller portion of the earth’s atmosphere. the scattering is much less and hence the sun looks White.

Question 42. A child sitting in a classroom is not able to read the writing on the blackboard.

1. Name the type of defect from which his eye is suffering.
2. With the help of a ray diagram show how this defect can be remedied.

Answer: The child’s eye is suffering from the defect of myopia.

The defect can be removed by using a concave lens as shown in the figure below:

Question 43. Why do we have two eyes instead of one?
Answer: Two eyes are better than one eye because of the following reasons:

• The field of view with two eyes is more than with one eye.
• Two eyes give three three-dimensional pictures of an object whereas one eye gives only two dimensional picture of an object.

Human Eye And Colourful World Multiple Choice Questions

Question 1. The human eye can focus objects at different distances by adjusting the focal length of the eye lens. This is due to

1. Presbyopia.
2. Accommodation.
3. Near-Sightedness.
4. Far-Sightedness.

Answer: 2. The ability of the eye to focus objects at different distances is called accommodation.

Question 2. The human eye forms the image of an object in its

1. Cornea.
2. Iris.
3. Pupil.
4. Retina.

Answer: 4. The human forms the image of an object at its retina.

Question 3. The least distance of distinct vision for a young adult with normal vision is about

1. 25 m.
2. 2.5 cm.
3. 25 cm.
4. 2.5 m.

Answer: 3. The least distance of distinct vision for a normal eye is 25 cm.

Question 4. The change in focal length of an eye lens is caused by the action of the

1. Pupil.
2. Retina.
3. Ciliary Muscles.
4. Iris.

Answer: 3. The change in the focal length of the eye is due to the action of ciliary muscles.

Question 5. A person cannot see distinct objects kept beyond 2m. This defect can be corrected by using a lens of power

1. + 0.5D
2. – 0.5 D
3. + 0.2 D
4. – 0.2 D

Answer: 2. – 0.5 D

Question 6. A student sitting on the last bench can read the letters written on the blackboard but is not able to read the letters written in his textbook. Which of the following statements is correct?

1. The near point of his eyes has receded away
2. The near point of his eyes has come closer to him
3. The far point of his eyes has come closer to him
4. The far point of his eyes has receded away

Answer: 1. The near point of his eyes has receded away

Question 7. A prism ABC (with BC as the base) is placed in different orientations. A narrow beam of white light is incident on the prism as shown in the figure. In which of the following cases, after dispersion, the third colour from the top corresponds to the colour of the sky?

1. 1
2. 2
3. 3
4. 4

Answer: 2. 1

Question 8. At noon the sun appears white as

1. Light Is Least Scattered
2. All The Colours of white Light Are Scattered Away
3. Blue Colour Is Scattered The Most
4. Red Colour Is Scattered The Most

Answer: 1. Light Is Least Scattered

Question 9. Which of the following phenomena of light are involved in the formation of the rainbow?

1. Reflection, Refraction And Dispersion
2. Refraction, Dispersion And Total Internal Reflection
3. Refraction, Dispersion And Internal Reflection
4. Dispersion, scattering and total internal reflection

Answer: 3. Refraction, Dispersion And Internal Reflection

Question 10. The twinkling of stars is due to atmospheric

1. Dispersion Of light By Water Droplets
2. Refraction Of Light By Different Layers Ofvarying Refractive Indices
3. Scattering Of light By Dust Particles
4. Internal Reflection of Light By Clouds

Answer: 2. Refraction Of light By Different Layers Ofvarying Refractive Indices

Question 11. The clear sky appears blue because

1. Blue Light Gets Absorbed In The Atmosphere
2. Ultraviolet Radiations Are Absorbed In The Atmosphere
3. Violet And Blue Lights Get Scattered More Than Lights Of All Other Colours By The
   Atmosphere
4. Light all other Colours Is Scattered More Than The Violet And Blue Colourlights By The Atmosphere

Answer: 3. Violet And Blue Lights Get Scattered More Than Lights Of All Other Colours By The Atmosphere

Question 12. Which of the following statements is correct regarding the-propagation oflight of different colours of white light in the air?

1. Red Light Moves the Fastest
2. Blue Light Moves Faster Than Green Light
3. All The Colours of white Light Move at the Same Speed
4. Yellow Light Moves With The Mean Speed As That Ofthe Red And The Violet Light

Answer: 3. All The Colours of white Light Move With The Same Speed

Question 13. The danger signals installed at the top of tall buildings are red. These can be easily seen from a distance because among all other colours, the red light

1. Is Scattered The Most By Smoke Or Fog
2. Is Scattered The Least By Smoke Or Fog
3. Is Absorbed The Most By Smoke Or Fog
4. Moves Fastest In Air

Answer: 2. Is Scattered The Least By Smoke Or Fog

Question 14. Which of the following phenomena Contributes Significantly To The Reddish Appearance Of The Sun At Sunrise Or Sunset?

1. Dispersion Of light
2. Total Internal Reflection Of light
3. Scattering Of light
4. Reflection Of Light From The Earth

Answer: 2. Total Internal Reflection Oflight

Question 15. The Bluish Colour Of Water In Deep Sea Is Due To

1. The Presence Of algae And other plants Found In Water
2. Reflection Ofsky In Water
3. Scattering Oflight
4. Absorption Oflight By The Sea

Answer: 3. Scattering Of light

Question 16. When Light Rays Enter The Eye, Most of the refraction Occurs At The

1. Crystalline Lens
2. Outer Surface Of the Cornea
3. Iris
4. Pupil

Answer: 2. Outer Surface Of the Cornea

Question 17. The Focal Length Of The Eye Lens Increases When Eye Muscles

1. Are Relaxed And Lens Becomes Thinner
2. Contract And Lens Becomes Thicker
3. Are Relaxed And Lens Becomes Thicker
4. Contract And Lens Becomes Thinner

Answer: 1. Are Relaxed And Lens Becomes Thinner

Question 18. Which of the following Statements Is Correct?

1. A Person With Myopia Can See Distant Objects Clearly
2. A Person With Hypermetropia Can See Nearby Objects Clearly
3. A Person With Myopia Can See Nearby Objects Clearly
4. A Person With Hypermetropia Cannot See Distant Objects Clearly

Answer: 3. A Person With Myopia Can See Nearby Objects Clearly

Question 19. Tarun’s father is an eye surgeon. He persuaded his father to put a camp in his society to educate people about eye donation. Rohan made a banner. One donation can give eyevision to two blind persons.

1. Name the part of the eye that is used, during eye transplant.
2. Name the defect that can be corrected by this transplant.
3. What value of Tarun is reflected?
4. The defect is caused by to cornea.

Answer: 1. Cornea.

1. A defect is caused by to cornea.
2. Tarun shows sympathy, compassion and empathy in his behaviour.