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NCERT Solutions for Class 8 Science Chapter 16 – Light

Orchid International School is pleased to offer the PDF download of NCERT Solutions for Class 8 Physics Chapter 16. These solutions are readily accessible on our official website. By downloading the PDF, you can access comprehensive NCERT Solutions, enabling you to thoroughly review Class 8 Science Chapter 16.

Light

Question 1 :

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

 

Answer :

 In a dark room, one cannot see things in a room, but things outside the room will be visible.

 


Question 2 :

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

 

Answer :

 

The difference between regular and diffused reflection is as follows:

Regular Reflection

Diffused Reflection

Regular Reflection takes place from a smooth and shiny surface.

Diffused Reflection takes place on a rough or irregular surface.

All rays are parallel after reflection.

The reflected rays are not parallel to each other.

Diffused reflection is not due to the failure of the laws of reflection. It is caused by irregularities in the reflecting surface.

 


Question 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

Answer :

a . A polished surface means the surface is very smooth, thus

regular reflection takes place.

b . A diffused reflection takes place on a chalk powder because its surface is uneven.

c . The surface of cardboard that reflects light is not smooth, thus diffused reflection will take place from a cardboard surface.

d .The water spread over marble makes the surface very

Smooth which makes it act like a plane surface. Hence, regular reflection takes place.

e .The surface of the mirror is very smooth and shiny. Therefore, regular reflection takes place.

f . A piece of paper may look smooth, but it has many irregularities on its surface. Thus, diffused reflection takes place.

 


Question 4 :

State the laws of reflection.

 

Answer :

 Laws of reflection can be stated as follows:

  1. The angle of reflection and angle of incidence is always equal to each other.

  2. The incident ray, the normal at the point of incidence and the reflected ray all lie in the same plane.

 


Question 5 :

Describe an activity to show that the incident ray, the reflected ray and the normal at the point of incidence lie in the same plane.

 

Answer :

To perform this experiment take a plane mirror and place it on a plane sheet of paper in a standing position with a block. Then draw an incidence line AB on this paper. Mark the points on the paper by carefully looking at the mirror. The line that goes after it appears on the screen. Draw a perpendicular on the mirror line and then remove the mirror. Join the points to make the reflected ray on the paper. You will observe that incident ray, reflected ray and normal will be in the same plane, i.e. on the sheet of paper. This verifies the law of reflection.

Law of Reflection

 


Question 6 :

Fill in the blanks in the following.

a. A person 

1

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.

Answer :

a .  A person 

1

1

 m in front of a plane mirror seems to be 

2

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 it in dim light.

d . Night birds have lesser cones than rods in their eyes.

 


Question 7 :

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

a. Always

b. Sometimes

c. Under special conditions

d. Never

 

Answer :

(a) Always

Since the first law of reflection states that the angle of incidence and

the angle of reflection is always equal to each other.

 


Question 8 :

The 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.

 

Answer :

b. Virtual, behind the mirror and of the same size as the object.

The image created by the plane mirror is built behind the mirror. This image cannot be created on screen, so it produces a virtual image of the size that of the object.

 


Question 9 :

Describe the construction of a kaleidoscope.

Answer :

The Kaleidoscope is made up of three rectangular squares each about 

15

15

 cm long and 

4

4

 cm wide which are joined together to build a prism. They are arranged in a circular cardboard tube. Note that the tube is slightly longer than the screen straps. One end of the tube is closed with a cardboard disc with a hole in the middle. To make the disk last longer, a piece of transparent plastic paper is attached to the bottom of the cardboard disk. At the other end, a round plate of the glass plane is fixed touching the mirrors.

Kaleidoscope

 


Question 10 :

Draw a labelled sketch of the human eye.

 

Answer :

 

Human Eye


Question 11 :

Gurmit wanted to perform Activity 

 using a laser torch. Her teacher advised her not to do so. Can you explain the basis of the teacher’s advice?

 

Answer :

The intensity of the laser light is very high which is harmful to the eye and can cause a permanent defect in the eye. She can lose her eyesight also due to severe damage to the retina. Therefore, it is advisable not to look at a laser beam directly.


Question 12 :

Explain how you can take care of your eyes.

 

Answer :

To protect our eyes, the following precautions should be taken:

i. Do not read in too little or too much light.

ii. Wash your eyes frequently with cold water.

iii. Do not read by bringing your book too close to your eyes or keeping it too far.

iv. Never rub your eyes.

v. If particles of dust go into our eyes, wash our eyes with clean water. If there is no improvement go to a doctor.

 


Question 13 :

What is the angle of incidence of a ray if the reflected ray is at an angle of 

90∘

90∘

Answer :

to the incident ray?

Ans: We know that angle of incidence is equal to the angle of reflection. Therefore, twice of incidence angle is equal to 

90∘

90∘

.

Thus, Angle of incidence 

i=

90

2

�=90∘2

 .

⇒i=

45

⇒�=45∘

 


Question 14 :

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

 

Answer :

Since the mirrors are kept parallel to each other, and Infinite no of images will be formed due to multiple reflections between them.

 


Question 15 :

Two mirrors meet at right angles. A ray of light is incident on one at an angle of 

30∘

30∘
as shown in Fig. 

16.19

16.19
. Draw the reflected ray from the second mirror.

Ray of Light

 

Answer :

 From the first law of reflection, The angle of reflection and angle of incidence is always equal to each other.

The diagram of the reflected ray from the second mirror is as follows:

Ray of Light


Question 16 :

Boojho stands at 

A


just on the side of a plane mirror as shown in Fig. 

16.20

16.20
. Can he see himself in the mirror? Also, can he see the image of objects situated at 

P,Q,R

�,�,�
?

Plane Mirror

 

Answer :

 

A

 cannot see his image as the length of the mirror is too short on his side. He can see the image of the objects at 

P

 and 

Q

 but cannot see the object at 

R

 as can be inferred from the ray diagram.

Plane Mirror

 


Question 17 :

Solve the below referring to the given diagram.

Plane Mirror

 

a. Find out the position of the image of an object situated at 

A

 in the plane mirror.

 

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?

 

Answer :

a . The image of 

A

 is formed behind the mirror at the same distance as that of 

A

 from the mirror.

Plane Mirror

b . Yes, Paheli at 

B

 can see the image.

c . Yes, Boojho at 

C

 can see the image.

d .  The image of 

A

 is unaffected by the movement of 

B

 and 

C

. Therefore, Paheli will be able to see the image of 

A

 while moving from 

B

 to 

C


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