Light - Exemplar Solutions 4

CBSE Class 8 Science
NCERT Exemplar Solution

20. Boojho planned an activity to observe an object A through pipes as shown in Fig. 16.8, so that he could see objects which he could not directly see.
(a) How many mirrors should he use to see the objects ?
(b) Indicate the positions of the mirrors in the figure.
(c) What must be the angle with respect to the incident light at which he should place the mirrors ?
(d) Indicate the direction of rays in the figure.
(e) If any of the mirrors is removed, will he be able to see the objects ?
Ans: (a) Three
(c) 45o.
(e) No, he will not be able to see the object if any of the mirrors is removed.

21. There is a mistake in each of the following ray diagrams given as Fig. 16.9 a, b, and c. Make the necessary correction (s).
Ans: According to the laws of reflection; angle of incidence is always equal to the angle of reflection. And the normal line is at 90to the plane. In all of these cases the figures should be as follows:

22. Explain the process which enables us to perceive motion in a cartoon film.
Ans: The cartoon film we see is actually the projection of static pictures on the screen in a specific order. Usually the static pictures are shown in a sequence at the rate of 24 pictures per second one after the other giving us the perception of movement.

23. How is the phenomenon of reflection used in making a kaleidoscope ? What are the applications of a kaleidoscope ?
Ans: In a kaleidoscope, three plane mirrors inclined at an angle of 60° to each other are joined together, and fixed in a tube. This operates on the principle of multiple reflections and used to find colourful patterns.
Applications of a kaleidoscope are as follows :-
Designers and artists use kaleidoscope to get ideas for new patterns to design wallpapers, Jewellery and fabrics.

24. Fig. 16.10 shows the word REST written in two ways in front of a mirror. Show how the word would appear in the mirror.
Ans: (A)

25. Write down the names of parts of the eye in the blank spaces shown in Fig. 16.10.
Ans: 1. Ciliary muscle
2. Iris
3. Lens
4. Cornea
5. Retina
6. Optic Nerve