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LIGHT
TEXTBOOK QUESTIONS AND THEIR ANSWERS
Q.1. Suppose you are in a dark room. Can you see objects in the
room? Can you see objects outside the room? Explain.
Ans. When we are in a dark room, we cannot see the objects in the
room because there is no light in the room and objects are
visible only when they reflect light from their surface.
We can see objects outside the room because objects outside
the room are in light. Light energy becomes off from the
surface of object and reaches our eyes and helps us to see the object.
Q.2. Differentiate between regular and diffused reflection. Does
diffused reflection means the failure of laws of reflection?
Ans. Regular reflection : When all the parallel rays reflected from
a plane surface are parallel to each other, it is called regular
reflection as shown in figure.
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Diffused reflection :
When all the parallel rays reflected from a surface are not
parallel, to each other the reflection is known as diffused
reflection or irregular reflection as shown in figure.
The diffused reflection is not due to the failure of laws of
reflection but caused by irregularities in the reflecting surface.
Q.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.
Polished wooden table, chalk powder, cardboard surface,
marble stone with water spread over it, mirror and a piece
of paper.
Ans. (a) Polished wooden table – It will have regular reflection as
its surface is smooth.
(b) Chalk powder – Chalk powder will have diffused
reflection as chalk particles are not smooth.
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(c) Cardboard surface – It will have diffused reflection as
the surface of the cardboard has minute irregularities.
(d) Marble stone with water spread over it – It will have
regular reflection if the water is still, and diffused
reflection if waves are formed on the water surface.
(e) Mirror – It will have regular reflection because of its
smooth surface.
(f) A piece of paper – It will have diffused reflection due to
minute irregularities on its surface.
Q.4. State the laws of reflection.
Ans. Laws of reflection states that
(a) Incident ray, reflected ray and normal drawn at the point
of incidence to the reflecting surface always lie in the
same plane.
(b) Angle of incidence is always equal to the angle of
reflection.
Q.5. 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.
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(b) If 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 __________ when you see
in dim light.
(d) Night birds have ______ cones than rods in their eyes.
Ans. (a) 2m,
(b) left, left,
(c) larger, (d) lesser
Q.6. Angle of incidence is equal to the angle of reflection
(a) Always
(b) Sometimes
(c) Under special conditions (d) Never
Ans. (a)
Q.7. 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.
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Ans. (b)
Q.8. Draw a labelled sketch of human eye.
Q.9. Gurmit wanted to using a laser torch. Her teacher advised
her not to do so. Can you explain the basis of the teachers
advise?
Ans. Teacher advised Gurmit not to put laser light using laser torch
in her friend’s eyes because laser light is harmful for the eyes
and can even cause permanent damage when thrown directly
into the eyes.
Q.10. Explain how you can take care of your eyes.
Ans. The following precautions should be taken in order to take
care of your eyes.
(a) We should always wash at least three times daily with
clean cold water.
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(b) We should not touch our eyes with dirty hands.
(c) We should not look at the sun with naked eyes.
(d) We should read books and watch television from a
suitable distance.
(e) No pointed objects should be brought near the eyes.
Q.11. What is the angle of incidence of a ray if the reflected ray is
at an angle of 90° to the incident ray?
Ans. If the reflected ray is at an angle of 90° to the incident ray, the
angle of incidence will be of 45°.
Q.12. How many images of a candle will be formed if it is placed
between two parallel plane mirrors separated by 40 cm?
Ans. If a candle is placed between two parallel plane mirrors
separated by 40cm, then infinite images will be formed.
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Q.13. Can you see an object in the dark?
Ans. No, we cannot see an object in the dark.
Q.14. How is seeing made possible?
Ans. When light rays get reflected from the object to the eyes,
seeing is made possible.
Q.15. How can you see the hairs at the back of your head?
Ans. We can see the hairs at the back of our head with the help of
two plane mirrors.
Q.16. What happens if two plane mirrors in combination is used?
Ans. If two plane mirrors are used in combination then multiple
images are formed.
Q.17. Can you explain how reflection from two mirrors enables
you to see objects which are not visible directly?
Ans. Reflection from two plane mirrors placed at an angle enable us
to see objects which are not visible directly. For example,
periscope consisting two plane mirrors, is used for seeing such
objects which are not visible directly.
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