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Light - Class 8 : NCERT Exercise Questions


Q.1 Suppose you are in a dark room. Can you see objects in the room? Can you see objects outside the room. Explain.
Sol. We are able to see any object, when the light reflected from the object reaches our eyes. In case of dark room, there is no light so objects cannot reflect any light. Hence, we are not able to see any object in the dark room. But, in case there is light outside the room, then we can see those objects as light gets reflected from these objects and reaches our eyes.

Q.2 Differentiate between regular and diffused reflection. Does diffused reflection mean the failure of the laws of reflection?
Sol. The differences between regular and diffused reflection are:

Regular reflection

Diffused reflection

Reflection from a smooth surface is called a regular reflection.

Reflection from a rough surface is called a diffused reflection.

The reflected rays in regular reflection moves in a parallel direction.

The reflected rays in diffused reflection moves in random direction.

For example: Reflection by a plane mirror.

For example: Reflection by a road 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.
(a) Polished wooden table                  (b) Chalk powder
(c) Cardboard surface                         (d) Marble floor with water spread over it
(e) Mirror                                             (f) Piece of paper
Sol. (a) Polished wooden table – Regular reflection
Polishing a wooden table will smooth its surface and will have no irregularities. Hence, regular reflection will take place.
(b) Chalk powder – Diffused reflection
Chalk powder when spread on a surface will result in an irregular surface. Hence, diffused reflection will take place.
(c) Cardboard surface – Diffused reflection
Cardboard surface is an irregular surface.  Hence, diffused reflection will take place.
(d) Marble floor with water spread over it – Regular reflection
Marble floor with water makes the marble a smooth surface. Hence, regular reflection will take place.
(e) Mirror – Regular reflection
Mirror is an example of smooth surface. Hence, regular reflection will take place.
(f) Piece of paper – Diffused reflection
A piece of paper is an example of irregular surface. It might look smooth, but there are many irregularities present on its surface. Hence, diffused reflection will take place.

Q.4 State the laws of reflection.
Sol. The laws of reflection are as follows:
(i) The angle of incidence is always equal to the angle of reflection.
(ii) The incident ray, the reflected ray and the normal to the surface at the point of incidence lie in the same plane.

Q.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.
Sol. Take a plane mirror and place it on the table. Now, take a paper sheet having a hole at its centre. Do take care that the light in the room is not bright. Hold the paper sheet normal to the table. Take another sheet and place it on the table such that it makes contact with the vertical mirror. Draw a normal line on the second sheet from the mirror. Now, pass light from a torch through the small hole such that the light ray 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. One can easily observe that the incident ray, the reflected ray and the normal to the mirror at point of incidence are on the sheet placed on the table. This proves that the incident ray, the reflected ray and the normal at the point of incidence lie in the same plane.  

Q.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) 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.
Sol. (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.

Q.7 Angle of incidence is equal to the angle of reflection
(a) Always                               (b) Sometimes
(c) Under special conditions   (d) Never
Sol. (a) Always

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

Q.9 Describe the construction of a kaleidoscope.
Sol. To construct a kaleidoscope, take three rectangular mirror strips about 15 cm long and 4 cm wide each. Join them together to form a prism. These are fixed in a circular cardboard like tube or tube of a thick chart paper. The tube must be slightly longer than the mirror strips. One end of the tube remains closed by a cardboard disc having a hole in the centre, through which one can see. To make the disc durable, paste a piece of transparent plastic sheet under the cardboard disc. At the other end, touching the mirrors, fix a circular plane glass plate. On this glass plate, several small 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.

Q.10 Draw a labeled sketch of the human eye.
Sol. The labelled sketch of the human eye is shown below:

Q.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 teachers advise?
Sol. The laser beam is harmful for the human eyes, since its intensity is very high. Hence, it can cause damage and lead to blindness. This is the reason one should not look at laser beam directly or indirectly.

Q.12 Explain how you can take care of your eyes.
Sol. Following are few points that can be taken into account to take care of your eyes:
(i) If there is any problem in eyes, one should consult an eye specialist.
(ii) Try to avoid reading in dim light and very bright light.
(iii) Never look at the sun or any powerful light source directly.
(iv) Do not rub your eyes.
(v) If any particle enters your eyes, wash your eyes with clean water.
(vi) Keep an average of atleast 25 cm between book and eyes while reading.

Q.13 What is the angle of incidence of a ray if the reflected ray is at an angle of 90° to the incident ray?
Sol. We know that angle of incident ray is equal to angle of reflected ray i.e. ∠i = ∠r
Here, it is given that ∠i + ∠r = 90°
⇒ ∠i + ∠i = 90°
⇒ 2∠i = 90°
⇒ ∠i = 45°

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

Q.15 Two mirrors meet at right angles. A ray of light is incident on one at an angle of 30° as shown in Fig. Draw the reflected ray from the second mirror.Sol. The figure is as shown below:

Q.16 Boojho stands at A just on the side of a plane mirror as shown in Fig. Can he see himself in the mirror? Also can he see the image of objects situated at P, Q and R?Sol. Boojho won’t be able to see his own image as he is not standing in the front of the mirror. However, he will be able to see objects at P and Q because the rays coming from P and Q will reflect from mirror and reach the Boojho’s eyes. While, the ray from object R will not reflect and reach Boojho’s eyes.

Q.17 (a) Find out the position of the image of an object situated at A in the plane mirror (Fig).
(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?

Sol. (a) Image of the object at position A will be formed behind the mirror. The distance of the image from the mirror is equal to the distance of A from the mirror.(b) Yes. Paheli at B can see the object.
(c) Yes. Boojho at C can see the object.
(d) If we trace the reflected rays from B and C backwards, they converge at point behind the mirror. Position of image A remains fixed even if Paheli moves.



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