1. Complete the figure. Write the practical application of it.

2. (a) A white saree with blue spots is viewed in red light how does it appear.
(b) Can the red letters on white paper distinguished in red light? Why?

(a) White saree appears as red.
      Blue spots appear as dark.
     [A red saree with dark spots]

(b) No. Red letters as well as white paper appear as red. So the letter cannot be identified.

3. (i) Suggest one way, in each case, by which we can detect the presence of:
    (a) Infrared radiations
    (b) Ultraviolet radiations.

(ii) Give one use of infrared radiations.

(i) It is the portion of spectrum:

(a) Just beyond the red end. A blackened thermometer shows a much larger rise of temperature when placed in the infrared region.
(b) Just before the violet end. These are detected by the silver chloride solution which turns violet and finally back in the ultraviolet light.

(ii) They are used in photography at night.

4. Why danger signals are red?

Danger signal are red because the red colour is scattered less on account of its longer wavelength, since I∝1λ4. The red colour can be seen even from far of distance.

5. How do you explain the blue colour of the sky?

We know that scattering of light by the atmosphere depends upon the colour and it is more for violet colour than for red colour. We receive more blue component after scattering of which light coming from the different part of the atmosphere. Thus, the sky gives us the impression of being blue.

6. Infrared radiations are used for photography in fog and dark.

Infrared radiations scatter much less than visible radiation, due to larger wavelength so can penetrate much more than visible light through particles of fog and in dark, to take photographs.

7. Why the quartz prism is used for ultraviolet spectrum not the glass prism?

Glass prism cannot allow to pass the ultraviolet radiation, but quartz prism allow to pass the ultraviolet radiation and due to refraction of ultraviolet radiation we can get the spectrum of ultraviolet radiation also.

8. Why is the white light polychromatic?

If we allow to pass white light into glass prism and screen is placed on other side of glass prism, we observe the spectrum of seven colours. It proves that white light contain waves of seven colours or wavelength. So, it proves that white light is polychromatic.

9. What is main difference between X-rays and gamma rays?

Gamma rays are emitted by radioactive nuclei. While X-rays are produced when electron jumps from a higher orbit to lower energy level (orbit), by striking energetic cathode rays or heavy atomic mass metal like tungsten.

10. What do you mean by primary colours?

The three colours of light namely blue, green and red can be mixed to produce any other colour of light. But these colours of light cannot be obtained by mixing other colours of light. Hence blue, green and red are the primary colours of light.

If blue, green and red lights. of the same intensity are mixed the result will be as follows.

Blue + Green = cyan
Red + Green = Yellow
Blue + Red = Magenta
Blue + Green + Red = white

11. (a) Signal lamps and Tail lamps of vehicles are reddish. Why?
(b) The head light of vehicle playing in high ranges have yellow light. What is the advantage of it?
(c) Infra red rays are used to take the clear photographs of very distant objects. Why?

(a) Red light having longer wavelength scatters less by dust, mist and air.l o the signal can be seen from a long distance.
(b) Yellow light having comparatively longer wave length scatters less in dust and mist. So light emitted by the head light is scattered less and it can pass beyond the dust and mist to illuminate the distant objects on the road.
(c) Infrared rays have a longer wave length. So it can pass through the atmosphere without much scattering. Hence the intensity of light doesn't get reduced much.

12. How will you investigate the existence of the radiation beyond the red and violet extremes of the spectrum?

If a blackened bulb, thermometer is moved from violet end towards the red end, first a steady rise in temperature is observed, but as the thermometer goes beyond the red end, there is rapid rise in temperature. This shows the existence of some kind of radiation producing the heating effect beyond the red end of the spectrum. If the radiations from the red end to the violet end are made to fall on the silver chloride solution, it almost remains unaffected beyond the violet end of the spectrum.

13. How do you explain the reddish appearance of the sun at sunrise and at sunset?

Both in the morning and in the evening i.e., at sunrise and sunset, when the sun is near the horizon, sunlight has to travel a much longer distance of the atmosphere. As a result, the light travelling from the sun loses most of the blue light due to scattering because its wavelength is small. The red light of long wavelength is scattered a little and reaches the observer. As a result, the sun appears to be red.

14.
 

Given above are 4 discs ABCD with different colours painted on it
    (a) Which are the discs having only complementary colours?
    (b) Which are the discs having only primary colours?
    (c) The discs are rotated very fast. What will be the colour obtained in the four discs?

(a) Disc A (Green and Magenta): Disc C (Red and Cyan)
(b) Disc B (Green and Red): Disc D (Blue and Red)
(c) Disc A - White
Disc B - yellow
Disc C - white
Disc D - Magenta