# NCERT Class 8 Science Chapter 13 ‘Light’: Notes and Solutions (Free PDF)

Well-written notes help us cover the chapter in its entirety and serve as a quick revision tool that becomes essential when preparing for our exams. Keeping the same in mind, we have prepared NCERT Solutions and Notes for Class 8 Science Chapter 13: Light. In the notes, you will find a summary of all important concepts and theories in the lesson. Also, you will come across key definitions in the chapter. As you proceed through the notes, you also get NCERT solutions to questions in the chapter and PDF for the same. Let’s start learning.

## NCERT Class 8 Science Chapter 13 Light: Notes

In NCERT Class 8 Science Chapter 13, we will learn about light, what are the laws of reflection, how the human eye works, etc. Let’s start with the chapter.

### What are the Laws of Reflection?

• The ray of light that strikes a surface, is known as the incident ray.
• The ray of light that comes back after getting reflected from the surface is known as reflected ray.
• A line drawn perpendicular to a surface is known as the normal.
• The angle between the normal and the incident ray is known as the angle of incidence.
• The angle between the normal and the reflected ray is known as the angle of reflection
• There are basically two laws of reflection:
• Features of an image formed on a plane mirror are:
1. The image formed is straight.
2. The image formed is the same distance as the object in front of the mirror.
3. The image formed is laterally inverted.
4. The image formed is of the same size as that of the object.
5. It is a virtual image.
• In an image formed by a mirror, the object’s left appears on the right and the right appears on the left. This is known as lateral inversion.

### What are Regular Reflection and Diffused Reflection?

• Regular reflection is defined as the reflection that takes on a smooth surface and the reflected rays all have the same angle of reflection and are parallel to each other.
• Diffused or irregular reflection is defined as the reflection that takes place on an irregular or rough surface. The reflected rays have different angles of reflection and are not parallel to each other.
• The objects which shine in the light of other objects are called illuminated objects. For Example: the Moon.
• The objects which emit their own light are known as luminous objects. For Example: The Sun.

### What is the Concept of Multiple Images?

• When two plane mirrors are placed parallel to each other, then the number of images formed is infinite. If not placed parallel to each other, then still multiple images are formed.
• This is because, after being reflected once, light can be re-reflected multiple times.
• This concept is used by periscopes that are found in submarines and kaleidoscopes also use this concept.

### The Human Eye

• Our eye is roughly spherical in shape.
• The outer coat of our eye is white and tough in nature to protect our eye from damage. Its transparent front part is known as the cornea
• Behind the cornea, we find a dark muscular structure called the iris. The iris is that part of the eye that gives it its distinctive colour
• In the iris, there is a small opening called the pupil. The size of the pupil is controlled by the iris.
• The iris controls the amount of light entering into the eye.
• Behind the pupil of the eye is a lens which is thicker in the centre.
• The lens focuses light on the back of the eye, on a layer called the retina.
• The retina contains several nerve cells. Sensations felt by the nerve cells are then transmitted to the brain through the optic nerve.
• There are two kinds of cells–
1. Cone Cells, which are sensitive to bright light. Cones sense colour.
2. Rod Cells, which are sensitive to dim light
• At the junction of the optic nerve and the retina, there are no sensory cells, so no vision is possible at that spot. This is called the blind spot.
• Eyelids prevent any object from entering our eyes.
• The most comfortable distance at which one can read with a normal eye is about 25 cm.
• Sometimes, particularly in old age, eyesight becomes foggy. It is due to the eye lens becoming cloudy. When it happens, persons are said to have cataract. There is a loss of vision, sometimes extremely severe.
• It is possible to treat this defect. The opaque lens is removed and a new artificial lens is inserted. Modern technology has made this procedure simpler and safer.

### How can we take care of our eyes?

• Have regular check-ups from an eye specialist.
• Wear spectacles.
• Do not look at the Sun or a powerful light directly.
• Always read at the normal distance for vision.
• One should include in the diet components which have vitamin A because it is good for our eyes. Raw carrots, broccoli, green vegetables (such as spinach) and cod liver oil are rich in vitamin A. Eggs, milk, curd, cheese, butter and fruits such as papaya and mango are also rich in vitamin A.

### How can Visually Impaired Persons Can Read and Write?

• Persons who are visually impaired try to identify things by touching and listening to voices more carefully.
• They develop their other senses more sharply.
• However, additional resources can enable them to develop their capabilities further.
• Non-optical aids include visual aids, tactual aids (using the sense of touch), auditory aids (using the sense of hearing) and electronic aids. Visual aids can magnify words, can provide suitable intensity of light and material at the proper distance.
• Tactual aids, including Braille writer slate and stylus, help the visually challenged persons in taking notes, reading and writing.
• Auditory aids include cassettes, tape recorders, talking books and other such devices.
• Electronic aids, such as talking calculators and computers, are also available for performing many computational tasks.
• Optical aids include bifocal lenses, contact lenses, tinted lenses, magnifiers and telescopic aids. While the lens combinations are used to rectify visual limitations, telescopic aids are available to view chalkboard and class demonstrations.

### What is the Braille System?

• The most popular resource for visually challenged persons is Braille.
• Louis Braille, himself a visually challenged person, developed a system for visually challenged persons and published it in 1821.
• The present system was adopted in 1932.
• There is Braille code for common languages, mathematics and scientific notation. Many Indian languages can be read using the Braille system.
• Braille texts can be produced by hand or by machine. Typewriter-like devices and printing machines have now been developed.

## Important Definitions in NCERT Class 8 Science Chapter 13: Light: Notes

Some important definitions covered in the  NCERT Class 8 Science Chapter 13 are mentioned below:

• Incident Ray: The ray of light that strikes a surface, is known as the incident ray.
• Reflected ray: The ray of light that comes back after getting reflected from the surface is known as reflected ray.
• Normal: A line drawn perpendicular to a surface is known as the normal.
• The angle of incidence: The angle between the normal and the incident ray is known as the angle of incidence.
• The angle of reflection: The angle between the normal and the reflected ray is known as the angle of reflection.
• Lateral inversion: In an image formed by a mirror, the left of the object appears on the right and the right appears on the left. This is known as lateral inversion.
• Illuminated objects: The objects which shine in the light of other objects are called illuminated objects. For Example:- the Moon.
• Luminous objects: The objects which emit their own light are known as luminous objects. For Example:- The Sun.
• Iris: Behind the cornea, we find a dark muscular structure called the iris.
• Blind Spot: At the junction of the optic nerve and the retina, there are no sensory cells, so no vision is possible at that spot. This is called the blind spot.

## NCERT Solutions of Class 8 Science Chapter 13: Light: Notes

Let us now explore solutions to NCERT Class 8 Science Chapter 13 questions. Get answers to all writing questions in the lesson ‘Light’ to understand the lesson and ace your Science exams.

Exercise Questions

1. Suppose you are in a dark room. Can you see objects in the room? Can you see objects outside the room? Explain.

Soln:

No, inside a dark room, it is not possible to see objects because there is no light which will result in no reflection of it from any object but outside the room., objects can be seen easily because there is light there that will reflect from the objects and enter our eyes. Hence, we will be able to see the objects.

2. Differentiate between regular and diffused reflection. Does diffused reflection mean the failure of the laws of reflection?

Soln:

The diffused reflection is not due to the failure of the laws of reflection. It is caused by the irregularities in the reflecting surface, like that of a cardboard

3. Mention against each of the following whether regular or diffused reflection will take place when a beam of light strikes. Justify your answer in each case.

(a) Polished wooden table

(b) Chalk powder

(c) Cardboard surface

(d) Marble floor with water spread over it

(e) Mirror

(f) Piece of paper

Soln:

4. State the laws of reflection.

Soln:

There are basically two laws of reflection:

• The angle of incidence is always equal to the angle of reflection.
• The incident ray, the normal, and the reflected ray, at the point of incidence, all lie in the same plane.

6. Fill in the blanks in the following.

(a) A person 1 m in front of a plane mirror seems to be _______________ m away from his image.

(b) If you touch your ____________ ear with your right hand in front of a plane mirror, it will be seen in the mirror that your right ear is touched with your ____________.

(c) The size of the pupil becomes ____________ when you see in dim light.

(d) Night birds have ____________ cones than rods in their eyes.

Soln:

(a) A person 1 m in front of a plane mirror seems to be 2 m away from his image.

(b) If you touch your left ear with your right hand in front of a plane mirror, it will be seen in the mirror that your right ear is touched with your left hand.

(c) The size of the pupil becomes large when you see in dim light.

(d) Night birds have fewer cones than rods in their eyes.

Choose the correct option in Questions 7 – 8

7. The angle of incidence is ________ equal to the angle of reflection.

(a) Always

(b) Sometimes

(c) Under special conditions

(d) Never

Soln:

(a) Always

8. Image formed by a plane mirror is

(a) virtual, behind the mirror and enlarged.

(b) virtual, behind the mirror and of the same size as the object.

(c) real at the surface of the mirror and enlarged.

(d) real, behind the mirror and of the same size as the object.

Soln:

(b) virtual, behind the mirror and of the same size as the object.

11. Gurmit wanted to perform Activity 16.8 using a laser torch. Her teacher advised her not to do so. Can you explain the basis of the teacher’s advice?

Soln:

The teacher said so because she knows that the density of the lase is very high and is harmful to human eyes. It can damage the retina. Hence it is not advised to look at a laser bean directly.

12. Explain how you can take care of your eyes.

Soln:

• Have regular check-ups from an eye specialist.
• Wear spectacles.
• Do not look at the Sun or a powerful light directly.
• Always read at the normal distance for vision.
• One should include in the diet components which have vitamin A because it is good for our eyes. Raw carrots, broccoli, and green vegetables (such as spinach) and cod liver oil are rich in vitamin A. Eggs, milk, curd, cheese, butter and fruits such as papaya and mango are also rich in vitamin A

13. What is the angle of incidence of a ray if the reflected ray is at an angle of 90° to the incident ray?

Soln:

45 Degrees.

14. How many images of a candle will be formed if it is placed between two parallel plane mirrors separated by 40 cm?

Soln:

In such a case the number of images formed will be multiple and infinite. This will be because of multiple reflections.

16. Boojho stands at A just on the side of a plane mirror, as shown in Fig. 16.20. Can he see himself in the mirror? Also, can he see the image of objects situated at P, Q and R?

Soln:

Boojho would not be able to see his image because the reflected ray would not reach his eyes. However, he will be able to see the images of objects P and Q because the rays coming from them would reach his eyes after getting reflected. But he won’t be able to see the image of R because the rays would not be deflected from the object R.

17. (a) Find out the position of the image of an object situated at A in the plane mirror (Fig. 16.21).

(b) Can Paheli at B see this image?

(c) Can Boojho at C see this image?

(d) When Paheli moves from B to C, where does the image of A move?

a) An image of an object that is placed at A will be formed behind the mirror. The distance of the image from the mirror will be equal to the distance of A from the mirror.

b) Yes, Paheli at B can see this image.

c) Yes, Boojho at C can see this image.

d) The image of the object at A will not move. The image will remain in the same place when Paheli moves from B to C.

## FAQs

Q1. What is the note of refraction of light Class 8?

Ans: When light travels from one medium to another and deviates from its original path, it is known as refraction.

Q2. Who discovered light?

Ans: Humphry Davy discovered light.
The two laws of reflection:
1st Law: The angle of incidence is always equal to the angle of reflection.
2nd Law: The incident ray, the normal, and the reflected ray, at the point of incidence, all lie in the same plane

Q3. What is the angle of incidence?

Ans: The angle between the normal and the incident ray is known as the angle of incidence.

Follow Leverage Edu for complete study material on CBSE Notes of Class 8 Science