CBSE Class 8 Science 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.

” Cbse Class 8 Science Chapter 13 Notes “

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

” Reflection Of Light Class 8 “

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.

” Light Class 8 Notes “

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.

” Laws Of Reflection Class 8 “

• 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

” Image Formation By Plane Mirror “

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.

” Human Eye Structure Class 8 “

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.

” Dispersion Of Light Into Vibgyor “

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.

” Kaleidoscope Working Principle “

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

” Cbse Class 8 Science Notes “

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.

” Regular And Diffused Reflection “

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

” Angle Of Incidence And Reflection “

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.

” Periscope Working Principle “

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.

” Types Of Reflection Class 8 “

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.

” Reflected Light And Its Properties “

• 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.