[7 marks Questions]

Question 17 Marks

Cross word puzzle :

Across:
3. The geometrical centre of a spherical mirror.
6. Centre of the sphere from which the mirror is made.
7. An optical device with a polished surface that reflects the light falling on it.
8. Image which can be formed on a screen.
9. Image which cannot be formed on a screen.
10. The bending of a light ray when it passes from one medium to another medium of different density.

Down:
1. Mirror which converges a parallel beam of light passing through it.
2. Imaginary line passing through the centre of curvature of the mirror.
4. Mirror which diverges a parallel beam of light passing through it.
5. The formation of rainbow is an example of ______.

Answer

Across:
3. Pole
6. Centre Of Curvature
7. Mirror
8. Real
9. Virtual
10. Refraction

Down:
1. Concave
2. Principal Axis
4. Convex
5. Dispersion

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Question 27 Marks

Explain the working of a periscope.

Answer

Periscope:

It is an instrument used for viewing bodies or ships, which are over and around another body or a submarine.
It is based on the principle of the law of reflection of light.
It consists of a long outer case and inside this case mirrors or prisms are kept at each end, inclined at an angle of 45°.
Light coming from the distant body falls on the mirror at the top end of the periscope and gets reflected vertically downward.
This light is reflected again by the second mirror kept at the bottom, so as to travel horizontally and reach the eye of the observer.
In some complex periscopes, optic fiber is used instead of mirrors for obtaining a higher resolution.
The distance between the mirrors also varies depending on the purpose of using

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