TEXTBOOK ANSWERS AND SOLUTIONS OF CBSE CLASS VIII SCIENCE Chapter 16 Light

1)Suppose you are in a dark room. Can you see objects in the
room? Can you see objects outside the room? Explain.
ANSWER:-If we are in a dark room, then it is not possible for us to see
objects in the room.
However, objects outside the room are visible to us as an object
becomes visible when light reaches our eye after being reflected from the
object.
2)Differentiate between regular and diffused reflection. Does
diffused reflection mean the failure of the laws of reflection?
ANSWER:-Regular reflection takes place from a smooth or a regular surface.
Irregular or diffused reflection takes place from
an irregular surface. In diffused reflection, the reflected
rays are not parallel to each other for parallel
incident rays. Each ray obeys the laws of reflection.
Therefore, laws of reflections are not violated in diffused or
irregular reflection
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) Polished wooden table → Regular reflection.
A polished surface is an example of a smooth surface. A polished
wooden table has a smooth
surface.
Hence, reflections from the polished table will be regular.
(b) Chalk powder → Diffused reflection
Chalk power spread on a surface is an example of an irregular
surface. It is not smooth. Therefore, diffused reflection will take place from
chalk powder.
(c) Cardboard surface → Diffused reflection
Cardboard surface is also an example of an irregular surface.
Hence, diffused reflection will
take
place from a cardboard surface.
(d) Marble floor with water spread over it → Regular reflection
Marble floor with water spread over it is an example of a regular
surface. This is because water makes the marble surface smooth. Hence, regular
reflection will take place from this surface.
(e) Mirror → Regular reflection
Mirror has a smooth surface. Therefore, it will give a regular
reflection.
(f) Piece of paper → Diffused reflection
Although a piece of paper may look smooth, but it has many
irregularities on its surface. Due to this reason, it will give a diffused
reflection.


4)State the laws of reflection.
ANSWER:-Laws of reflection:
(i) The angle of reflection is always equal to the angle of
incidence.
(ii) The incident ray, the reflected ray and the normal to the
reflective surface at the point of
incidence all lie in the same plane.
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:-Place a plane mirror on the table. Take a paper sheet and make a
small hole in its centre. Make sure that the light in the room is not bright.
Hold the sheet normal to the table. Take another
sheet and place it on the table in contact with the
vertical mirror. Draw a normal line on the second sheet from the mirror. Now,
light a torch on the mirror through the small hole such that the ray of light
falls on the normal at the bottom of the mirror. When the ray from this hole is
incident on the mirror, it gets reflected in a certain
direction. You can easily observe the incident ray, reflected ray and the
normal to the mirror at the point of incidence on the sheet placed on the table. This shows that the incident ray, the
reflected ray, and the normal to the surface
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 __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 __less__ cones than rods in their eyes.
7) Angle of incidence is equal to the angle of reflection.
(a) Always (b) Sometimes
(c) Under special conditions (d) Never
ANSWER:- (a) The angle of incidence is always equal to the
angle of reflection.


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 itmirror and of the same size as the object.
ANSWER:- (b) Image formed by a plane mirror is virtual, behind the mirror
and of the same size as the object.
9)Describe the construction of a kaleidoscope.
ANSWER:-Three rectangular mirror strips of dimensions
15cm × 4cm (l × b) are joined together to form a
prism. This prism is fixed into a circular cardboard
tube. The circular cardboard tube should be slightly longer that the prism .This circular tube is now closed at
one end with a cardboard disc.
This disc has a hole in it through. At the other end of the
circular tube, a plane glass plate is fixed. It is important that this glass
plate touches the prism mirrors. On this glass plate, several small and broken
pieces of coloured glass are placed. This end is now

closed by a round glass plate allowing enough space
for the coloured glass pieces to move.
10)Draw a labelled sketch of the human eye.
ANSWER:-

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?
ANSWER:-Laser light is harmful for the human eyes, because its intensity
is very high. It can cause
damage
to the retina and lead to blindness. Hence, it is advisable not to look at a
laser beam directly.


12)Explain how you can take care of your eyes.
ANSWER:-To protect our eyes, the given points should be taken into
account:
(i) Visit an eye specialist regularly.
(ii) Avoid reading in dim light and very bright light.
(iii) Avoid direct exposure of sunlight to the eye.
(iv) Clean your eyes with cold water quickly if dust particles or
small insects enter your eye. Do
not rub your eyes.
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:-If the reflected ray is at an angle of 90° to the incident ray,
then the angle of incidence is 45 °.
14)How many images of a candle will be formed if it is placed
between two parallel plane mirrors separated by 40 cm?
ANSWER:-Infinite or multiple images of the candle will be
formed because of multiple reflections between the
mirrors.
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:-

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?

ANSWER:-A plane mirror forms a virtual image behind the mirror. The image
is as far behind the mirror as the object is in front of it. A cannot see his
image because the length of the mirror is too short on his side. However, he
can see the objects placed at points P and Q, but cannot see

the object placed at point 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) Image of the object placed at A is formed behind the mirror.

b) Yes. Paheli at B can see this image.
(c) Yes. Boojho at C can see this image.
(d) Image of the object at A will not move. It will remain at the
same position when Paheli moves
from B
to C.