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1. Objects present in the dark room are not visible. But when the light is switched on, everything present in the room becomes visible. Why does this happen?

We can see the objects only when the light reflected from the object is get focused on the retina of our eye. In the absence of light the image of the object is not formed and we cannot see the objects.

2. Differentiate between regular and irregular reflection.

The bouncing of light from a surface of a body is known as reflection.

Reflection from a polished surface is called regular reflection. In regular reflection the angle of incidence will be equal to angle of reflection.

Reflection from a rough or unpolished surface is called irregular or diffused reflection. In the case of a rough surface, light is not reflected in one direction. It is scattered in all directions.

3. State the laws of reflection.

The laws of reflection are,

(a) The incident ray, the reflected ray and the normal to the surface at the point of incidence lie in the same plane.

(b) The angle of incidence is equal to angle of reflection.

Li = Lr

4. Explain the working of a mirror periscope.

The working of a mirror periscope is based on the principle of successive reflections from two plane mirrors. It consists of two plane mirrors facing each other fixed at 45º to the frame work of a tube. The light ray from the object is get reflected from the first mirror and incident on the second mirror. The reflected image from the second mirror gives the actual image of the object.

5. Define refraction of light ray?

When light ray pass from one medium to another medium, the path of the ray will be changed. This is called refraction of light ray.

6. What are the laws of refraction?

When light travels from a rarer medium and enters a denser medium, it will be deviated towards the normal.

The light will be deviated away from the normal when it passes from a denser into a rarer medium.

7. Define total internal reflection?

If the angle of incidence is more than the critical angle, the ray bends inside the denser medium it self and this phenomenon is called total internal reflection.

The angle of incidence for which angle of refraction becomes 90º is called critical angle.

8. What are the conditions for total internal reflection?

1. The light must proceed from a denser medium to a rarer medium.
2. The angle of incidence in the denser medium must be greater than the critical angle.

9. What do you mean by power of accommodation of human eye?

The human eye focuses the image for different objects at different distances by changing the focal length of the lens. This is done by the Ciliary muscles, which stretch and relax to change the focal length of the lens. This action of the eye is called the power of accommodation of the eye.

10. Write an experiment to prove that sound can travel through liquid?

take two pebbles or marbles and beat them together and we can heard the sound. Then submerge them in a bucket of water and then tap the pebbles together under water. By keeping our ear near the bucket we can listen the sound. Hence we can prove that sound can travel through liquid.

11. For traveling sound needs a medium. Explain?

A sound producing cell phone is placed inside a bell jar and is connected to a vacuum pump. When we remove the air from the bell jar using the vacuum pump the sound gradually gets feebler. When the entire air is removed from the jar and made vacuum, we cannot hear the sound from the cell phone. From this we can under stand that second cannot travel through vacuum and it needs a medium to propagate.

12. Define frequency?

The number of oscillations per second is called the frequency. Frequency is expressed by hertz.

13. Explain the relation between time period and frequency?

The time taken by a vibrating body to complete one vibrations or oscillation is called the time period. While frequency is number of oscillations per second.

14. Define amplitude.

The maximum displacement of a vibrating body from its mean position is called amplitude. The unit is metre (m). The loudness of the sound depends on its amplitude.

15. Suggest some measures to reduce noise pollusion in your residential area.

1. The use of loudspeaker in functions should be stopped.
2. T.V and musical systems should be listened at low volumes.
3. Plant trees.

16. If Raman fixed two mirrors at an angle of 60º to get as many number of images, could you find out the exact number of images produced?

17. Veena and Rani are on moon, Veena calls out her friend but Rani does not hear Veena's call even though she is near. Why?

In moon there present no air. morder to travel sound needs medium. In vacuum sound do not travel. Thats why Rani does not heard the call of Veena.

An optical fibre is a device based on the principle of total internal reflection. They are then flexible and transparent strands of glass which can carry light along them. When light is incident at one end of the fibre at a small angle the light that passes inside undergoes repeated total internal reflections along the fibre and finally comes out.

19. What are the uses of optical fibres?

1. Optical fibres are used to transmit communication signals.
2. In medicine, optical fibres are used in endoscope and laporoscopes.

20. Differentiate between audible and maudible sounds.

The human ear can hear the range of audible frequencies between 20 Hz and 20000 Hz. They are called audible sounds.

Sounds of frequencies lesser than 20Hz and greater than 20000Hz can not be heard by the human ear. They are called maudible sounds.

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##### Practice in Related Chapters
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Force and Pressure
Electricity and Heat
Light and Sound