Light class 8th ncert solutions

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Exercises
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                              LIGHT CLASS 8TH                     
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:
We can see any object, when light reflected by that object reaches to our eyes. But in the dark room,
no light is retlected by the object. Hence, we are not able to see the objects in dark room. But if there
is light outside the room, we can see the objects lying out there.

Question 2:
Differentiate between regular and diffused retlection. Does diffused reflection mean the failure of the
laws of reflection?
Answer 2:
S. No. Regular Reflection

Diffused Reflection

1.

It takes place from a smooth place.

It takes place from a rough surtace.

Retlected rays moves in a particular Reflected rays scattered in random
direction.

Example: Reflection by plane mirTOr.

Retlection by road surface.

Rins neeesed from epilnr schan

Riaus refleeted Jro

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
() Cardboard surface
e) Mirror
Answer 3:
(a) A polished wooden table is an example of smooth surface. So, when a beam of light reflects from
it, a regular reflection will take place.
(b) Chalk powder has irregular surface. So, when a beam of light reflects from it, a diffused reflection
will take place.
()A cardboard has irregular surface. So, when a beam of light reflects from it, a diffused reflection
will take place.
(d) A marble floor with water spread over it provides a smooth surface. So, when a beam of light
reflects from it, a regular reflection will take place.
(e) A mirror has a smooth surface. So, when a beam of light reflects from it, a regular reflection will
take place.
(0Apiece of paper may appear smooth but it has many minor irregularities. So, when a beam of light
reflects from it, a diffused reflection will take place.

3:

(b) Chalk powder
(d) Marble floor with water spread over it
0 Piece of paper



Question 4:
State the laws of reflection.
Answer

Laws of reflcction:

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

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

Question 5:

Describe an activity to show that the incident ray, the retlected ray and the normal at the point of
incidence lie in the same plane.
Answer 5;
Fix a white sheet of stiff paper on a drawing board ora table. Take a comb and close all its openings

except one in the middle. Hold the comb perpendicular to the sheet of paper. Throw light trom a torch
through the opening of the comb from one side. With slight adjustment of the torch and the comb we
will see a ray of light along the paper on the other side of the comb. Keep the comb and the torch
steady. Place a strip of plane mirror in the path of the light ray. Let the sheet project a little beyond
the edge of the Table. Cut the projecting portion of the sheet in the middle. Look at the reflected ray.
Make sure that tne
ray extends to the projected
portion ot the paper. Bend
that part of the projected
portion on which the
reflected ray falls. Bring the
paper back to the original
position.
When the whole sheet of
paper is spread on the table, it
represents one plane. The
incident ray, the normal at the
point of incidence and the reflected ray are al in this plane. When we bend the paper you create a
plane different from the plane in which the incident ray and the normal lie. Then we do not see the
retlected ray. It indicates that the incident ray, the normal at the point of incidence and the reflected

reiecteu

Question 6:
Fill in the blanks in the following:
(a) A person 1 m in front of a plane mirror seems to be

m from his image.

(b) It you touch your

ear with right hand in front of a plane mirror it will be seen in the

mirror that your right ear is touched with-
(c) The size of the pupil becomes.
a Night birds have

when you see in dim light.

cones than rods in their eyes.

Answer 6:

(a) A person 1 m in front of a plane mirror seems to be 2 m from his image.
(b) If you touch your left ear with right hand in front of a plane mirror it will be seen in the mirror
that your right ear is touched with left hand.
(C) The size of the pupil becomes large when you see in dim light.
(d) Night birds have less cones than rods in their eyes.

Question 7:

Choose the correct option: Angle of incidence is equal to the angle of reflection

(a) Always

(b) Sometimes

() Under special conditions

d)

Never

Answer 7:

a) Always

Question 8:
Choose the correct option: 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 mirTor and enlarged
(d) real, behind the mirror and of the same size as the object
EAnswer 8:

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

Question 9:

Describe the construction of a kaleidoscope.

Answer 9:
are fix in a circular cardboard like tube or tube or a thick chart paper. The tube is slighty longer than
through which one can see. To make the disc durable, a piece or transparent plastc Sheet is pasted
On this glass plate several small pleces of coloured glass is placed to form multiple images.

A kaleidoscope is made up of three rectangular mirror strips joined together to forma prism. These

r strips. One end of the tube remains closed by a cardboard disc having a hole in the centre,

e cardboard disc. At the other end, touching the mirrors, a circular plane gass plate is fixed.

Cairwctiom of Kalaidnoe

Question 10:

Draw a labelled sketch of the human eye.
 Answer 10: Make itself

Question ll:
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?
Answer 11:
Intensity of laser beam is very high, as it carries large amount of energy. It is harmful for eyes and
can cause permanent damage. One should not look at laser beam directiy or indirectiy tor a longer
period.

Question 12:

EXplain how you can täke care ot your eyes.

Answer 12:
It is necessary that we take proper care of our eyes. If there is any problem we should go to an eye
specialist. Have a regular check-up.

lf advised, we should use suitable spectacles.
Too ittle or too much light s bad for eyes. Insufficient light causes eyestrain and headaches.

Too much light, like that of the sun, a powerful lamp or a laser torch can injure the retina.
We should not look at the sun or a powertul light directly.
Never rub our eyes. If particles of dust go into our eyes, wash our eyes with clean water. I

there is no improvement go to a doctor.

Wash our eyes frequently with clean water

Always read at the normal distance for vision. Do not read by bringing your book too close to

Our eyes or keeping it too far.

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

Answer 13:
We know that angle of incident ray is equal to angle of reflected ray.

But it is given that

Li + r = 90
Li+ 2i = 90°
22I90°

Li =-

=4

Question 14:

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

Intinite number of images.

Question 15:
Two mirrors meet at right angles. A ray of light is incident
on one at an angle of 30" as shown in Fig 16.19. Draw the
reflected ray from the second mirror.


Answer 15:Draw itself

Question 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

ABooho)

objects situated at P,Qand R?

Answer 16:

Boojho can't see his own image because he is not standing in tront of the mirror. However, he is al
to see objects at P and Q because the rays coming from P and Q get reflected by mirror and reach
his eyes. The ray from object R does not refiect and reaches Boojho, so, he cannot see object R.

A(Boojho)

IWARI

Question 17:
(a) Find out the p0sition of the image of an object

.B (Paheli)

situated atA in the plane mirror (Fig. 16.21).
(b) Can Paheli at B see this image?
() Can Boojho at C see this image?
(d When Paheli moves from B to C, where does the
image of A move?

C (Boojho)

mm

m

Answer 17:

a) The image of the object at position A will be formed
behind the mirror. It will be at the same distance away
from mirror as the object is.

(D res Piahell at B can see the object because reflected
Wil reach B.
C) Boojho can also see the image because his eyes
receives the reflected ray from A.

, B (Paheli)
c(Boojho)

a we trace the rellected rays from B and C
backwards, they converge at point behind the mirror
Position of image A remains fixed even if Paheli moves
from B to C.

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