Question

4. Assertion-Reason Type Questions:
Note: Use instructions as given in exercises of the chapter
1) A Twinkling of stars is due to atmospheric refraction.
R. Blue colour of sky is due to scattering of blue light. It is called Tyndall effect.
2) A. Sun appears reddish in the morning. It will not be observed on moon.
R. It is because moon does not have atmosphere, therefore, light will not scatter.​

Answer 1

Explanation:

1 A. The twinkling of a star is due to atmospheric refraction of starlight. The starlight, on entering the earth's atmosphere, undergoes refraction continuously before it reaches the earth.

R - Blue colour of the sky is due to scattering of light by colloidal dust particles present in air (Tyndall effect).

2 A . The long wavelength of the light that comes through the Earth is red colour. This is the reason sun appear reddish early in the morning. The astronaut will not observe this phenomenon on the moon's surface, because the moon has no atmosphere. Therefore no light will be distributed, and darkness will arise.

R - During the sunrise the sun is set close to the Earth's horizon, and therefore light travels a longer distance through the atmosphere of the Earth. ... The astronaut will not observe this phenomenon on the moon's surface, because the moon has no atmosphere. Therefore no light will be distributed, and darkness will arise.

Answer 2

Atmospheric refraction is the deviation of light from a straight line as it passes through the atmosphere due to the variation in density of the air.

Explanation:

• The twinkling of a star is caused by starlight being refracted by the atmosphere. The refraction takes place in a medium with a changing refractive index. Because the physical conditions of the earth's atmosphere are not stationary, the apparent position of the star does not remain constant. The apparent position of the star fluctuates as the path of the rays of light coming from the star varies slightly, and the amount of starlight entering the eye flickers- the star appears brighter at times and fainter at other times, which is the twinkling effect. Hence, The Assertion is True.
• Gases and particles in Earth's atmosphere scatter sunlight in all directions. Blue light is scattered more than other colors because it travels as shorter, smaller waves. This is why we see a blue sky most of the time. Hence, The Reason is also True but it is not the Correct Explanation of the Assertion.

Therefore, Here A and R both are true but R is not the correct explanation for A.

• The light rays from the Sun must travel a greater distance through the earth's atmosphere before reaching our eyes at sunrise. Because blue has a shorter wavelength and red has a longer wavelength, the red colour is able to reach our eyes after the light is scattered by the atmosphere. Hence, The Assertion is True.
• Since, Moon has no atmosphere, no scattering of light by the particles will be observed. As a result, sun will not appear to be red. Hence, The Reason is Also Tue and is the correct explanation for the assertion.

Therefore, Here A and R both are true and R is the correct explanation for A.