1. Fill in the blanks. 

(a) When the beams from the headlights of a car fall on an object in the night, the shadows called ………….. and …………… can be seen.

Ans: umbra and penumbra.

(b) During a lunar eclipse the shadow of the ………… falls on the ………… .

Ans: earth, moon.

(c) During a solar eclipse the shadow of the ………….. falls on the ………….. .

Ans: moon, earth.

(d) Various shades of colour are seen in the sky at sunrise and sunset due to ………… .

Ans: scattering of light.

2. Give reasons. 

(a) Space beyond the earth’s atmosphere appears dark.

Ans: 

(i) When sunlight is scattered by the particles present in the atmosphere, the surrounding gets illuminated and appears bright. 

(ii) In space, there is no atmosphere. Hence light doesn’t get scattered at all while in space. 

(iii) There are no air particles against which the sunlight can strike, get reflected or scattered. 

(iv) Hence space beyond the earth’s atmosphere appears dark.

(b) We are able to read while sitting in the shade.

Ans: 

(i) The sun is an extended source of light that is far away from the earth. 

(ii) The sunlight casts a shadow on the object on the earth in its path. 

(iii) This shadow has no umbra but a penumbra, which is nothing but the shade. 

(iv) There is a sufficient amount of light available to read in the shade. 

(v) Hence, we are able to read while sitting in the shade.

(c) We should not observe the solar eclipse with naked eyes.

Ans: 

(i) The earth receives the harmful ultraviolet rays from the sun. 

(ii) During a solar eclipse, even if the brightness of the sun is reduced, the ultra-violet rays still reach the earth. 

(iii) If the solar eclipse is seen or watched with naked eyes, the ultraviolet rays directly enter our eyes and damage our vision. 

(iv) Hence, the solar eclipse should not be observed with naked eyes.

3. Give some examples of scattering of light that we come across in day-to-day life. 

Ans:

(i) A beam of light emerging from a cinema projector on a screen

(ii) A beam of light emerging from the headlamps of a car moving through a thick fog

(iii) The sun appears red at sunset.

(iv) The sky appears blue.

4. Why is the shadow of a bird flying high not seen on the earth?

Ans:

(i) The sun is an extended source of light that is extremely bigger than the earth. 

(ii) When a bird flies (at a relatively shorter height from the ground), there is a formation of umbra and penumbra on the ground. 

(iii) But, as they fly high in the sky, their umbra becomes smaller and smaller and eventually disappears. 

(iv) At the same time, their penumbra becomes more and more fainter and then disappears. 

As a result, the shadow of the bird flying high is not seen on the earth.

5. Why is a penumbra not obtained from a point source? 

Ans: Scattering of light emerging from a point source does not take place. Thus, only dark shadow of the object placed in front of the point source is obtained.

6. Answer the following questions in your own words. 

(a) What is meant by scattering of light ?

Ans: The light rays hit the molecules, dust particles, and other tiny particles present in the atmosphere and get scattered. This phenomenon is called the scattering of light. When the scattered light rays enter our eyes, we perceive the light. A beam of light, blue sky, and the reddish appearance of the sun are all effects of the scattering of light.

(b) Does the shadow really vanish in the zero shadow condition?

Ans: On a day with zero shadow, at noon, the sun reaches exactly over our heads. Thus, the shadow of our body in a standing position (without stretching hands and legs) is formed right below the base of our feet. The shadow so formed is not seen, and hence appears to have vanished.

(c) Will the laser beam be seen if it passes through a glass box which contains a lighted incense stick?

Ans: A scattering of light is required for a ray of light to be visible. For this, the light rays should get scattered upon hitting the tiny particles in their path. There are tiny particles of the incense stick scattered in the closed glass box, due to which the laser rays get scattered. These scattered rays enter our eyes, and the laser beam is seen.

7. Discuss and write. 

(a) Write a science based paragraph on ‘What if the sun did not rise ?’

Ans: If the sun does not rise, the earth will not be illuminated. Due to this, we won’t be able to see objects around us if any other temporary source of light is not used. Also, as we know, the sun is the main source of energy for all living beings. So, plants won’t be able to prepare their food because of the absence of sunlight, and soon they will die. This will create an imbalance in the food chain, which in turn will result in the destruction of all kinds of organisms on Earth. Also, all the production of electrical energy will cease because of the absence of this solar energy. Thus, it is true to this day that the sun is the main source of our survival.

(b) What efforts will you make to remove the misconceptions about eclipses ?

Ans: We will make people understand and accept the fact that an eclipse is a natural phenomenon like rain, the spectrum, and seasons. For which, we will take the following steps:

(i) We will provide scientific information about eclipses through various media.

(ii) If an eclipse is going to take place in the near future, we will create awareness about it through advertisements.

(iii) We will provide the experience of viewing an eclipse through special goggles and telescopes.

(iv) We will arrange a tour to observe an eclipse taking place in a region far away from us.

(v) We will show people how an eclipse takes place by making models of the sun, the moon, and the earth.

(vi) We will convince people that the intake of food during the period of the eclipse does not cause any ill effects on health.

(c) Various eclipses and the conditions during that period. 

Ans:

(1) A solar eclipse:

The moon comes between the sun and the earth in a straight line. Thus, the shadow of the moon falls on the earth. 

a. A total solar eclipse 

(i) It is visible from within the part of the earth where the umbra of the moon falls. 

(ii) The solar disc is completely covered by the moon. 

(iii) Darkness spreads on the part of the earth where the shadow of the moon falls.

b. A partial solar eclipse 

(i) It is visible from within the part of the earth where the penumbra of the moon falls. 

(ii) The moon does not cover the solar disc completely. 

c. An annular solar eclipse 

(i) It is seen or visible from the part of the earth where a very small portion of the umbra of the moon falls. 

(ii) It is seen that the entire solar disc, except the peripheral ring, is completely covered by the moon. 

(iii) The edge of the solar disc appears like an illuminated ring. 

(2) A lunar eclipse:

The earth comes between the sun and the moon in a straight line. Thus, the shadow of the earth falls on the moon. shadow of the

a. A total lunar eclipse:

At this event, the moon completely comes into contact with the earth.

b. A partial lunar eclipse:

At this event, a part of the moon comes into the shadow of the earth.

8. Explain the difference :

(a) Point sources and extended sources of light. 

Ans: