CBSE Class 8 Science Light VBQs

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 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. Choose the correct option in Questions 7 – 8

Question. Angle of incidence is equal to the angle of reflection.
(a) Always (✓)
(b) Sometimes
(c) Under special conditions
(d) Never
Answer : A

Question 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.
Answer : B

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.
Answer : Regular reflection will take place as the surface is smooth.

(b) Chalk powder.
Answer : Diffused reflection will take place as the surface is not smooth.

(c) Cardboard surface.
Answer : Diffused reflection will take place as the surface is not smooth.

(d) Marble floor with water spread over it.
Answer : Regular reflection will take place as the surface is smooth.

(e) Mirror.
Answer : Regular reflection will take place as the surface is smooth.

(f) Piece of paper.
Answer : Diffused reflection will take place as the surface is not smooth.

Question. 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 objects only if light from any source enter our eyes. If we are in a dark room, then it is not possible for us to see objects in the room. However, if there is light outside the room, then objects outside the room are visible to us.

Question. 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 at the point of incidence, all lie in the same plane.

Question. How many images of a candle will be formed if it is placed between two parallel plane mirrors separated by 40 cm?
Answer : Infinite number of images.

Question. 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 : Fix a white sheet of paper on a drawing board or a table. Let the sheet project a little beyond the edge of the table. Make a ray of light by passing torch light through a comb with all its openings closed except one in the middle. Place a strip of plane mirror in the path of the light ray. After striking the mirror, the ray of light is reflected in another direction. This is called the reflected ray. Now, cut the projecting portion of the sheet in the middle. Look at the reflected ray. Bend that part of the projected portion on which the reflected ray falls. The reflected ray that fell along this part can no longer be seen. This shows that the incident ray, the reflected ray and the normal at the point of incidence lie in the same plane.

Question. Describe the construction of a kaleidoscope.
Answer : Take three rectangular mirror strips of dimensions 15cm x 4cm and join them together to form a prism. A prism is fixed into a circular cardboard tube. This circular tube is now closed at one end with a cardboard disc. This disc has a hole through which we can see. At the other end of the circular tube, a plane glass plate is fixed. 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.

Question. What is the angle of incidence of a ray if the reflected ray is at an angle of 90° to the incident ray?
Answer : 45º.

Question. 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 advise?
Answer : Because of the intensity of the laser light is very high, and it is harmful to the human eyes. It can cause damage to the retina and leads to blindness.

Question. Differentiate between regular and diffused reflection. Does diffused reflection mean the failure of the laws of reflection?
Answer :

The laws of reflection have not failed even in diffused reflection, because each ray obeys the law of reflection.

Question. Explain how you can take care of your eyes.
Answer : (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.
(v) Do not rub your eyes.
(vi) Maintain a distance of at least 25 cm between the book and your eyes while reading.

Question. Draw a labelled sketch of the human eye.
Answer :

Question. (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?
Answer : a) Image of an object placed at A is formed behind the mirror at the same distance.
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.