NCERT Solutions for Class 8 Science Chapter 16 – Light

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

 

 

The difference between regular and diffused reflection is as follows:

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

 

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.

 

 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.

 

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.

 

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.

 

(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.

 

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.

 

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.

 

 

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.

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.

 

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∘

 

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

 

 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:

 

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.

 

a . The image of 

A

�

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

A

�

 from the 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

�