Class 8 Science

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.

Read And Learn More CBSE Class 8 Science Notes

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.

Read And Learn More CBSE Class 8 Science Notes

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.

Read And Learn More CBSE Class 8 Science Notes

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.

• Chapter 1 Crop Production and Management Notes
• Chapter 2 Microorganisms: Friend and Foe Notes
• Chapter 3 Coal and Petroleum Notes
• Chapter 4: Combustion and Flame Notes
• Chapter 5: Conservation of Plants and Animals Notes
• Chapter 6 Reproduction in Animals Notes
• Chapter 7: Reaching the Age of Adolescence Notes
• Chapter 8 Force and Pressure Notes
• Chapter 9 Friction Notes
• Chapter 10 Sound Notes
• Chapter 11 Chemical Effects of Electric Current Notes
• Chapter 12: Some Natural Phenomena Notes
• Chapter 13 Light Notes

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.