Question

Give reasons for the following:
(i) The sky appears to be blue during day time to a person on earth.
(ii) The sky near the horizon appears to have a reddish hue at the time of sunset and sunrise.
(iii) The sky appears dark instead of blue to an astronaut.
(iv) The stars appear to twinkle.
​(v) The planets do not twinkle.

Answer 1

(i) The scattered light's color depends on the size of the scattering particles. The particles of very small size scatter mainly blue light whereas, the particles of larger size scatter lights of longer wavelengths. The sky appears blue because the fine particles present in the earth's atmosphere scatter blue light the most among all the components of white light.

(ii) At sunrise or sunset, the Sun is located near the horizon. So, the light has to travel a long distance through the Earth’s atmosphere. At the time of sunrise or sunset, when white sunlight falls on suspended atmospheric particles, the blue color light scatters out in the atmosphere, while the red color light scatters less and reaches the eyes of the observer on the surface of the Earth. Hence, when this less scattered red light reaches the eyes of the observer, the Sun and its surroundings appear reddish.

(iii) The sky appears dark instead of blue to an astronaut because there is no atmosphere in the outer space that can scatter the sunlight. As the sunlight is not scattered, the sky appears black to them.

(iv) The light coming from the stars undergoes refraction through the earth’s atmosphere. The physical conditions of the earth’s atmosphere keep changing. As a result of that, the path and also the amount of refracted starlight changes continuously through the atmosphere. Therefore, the stars sometimes appear brighter and sometimes fainter. As a result of this, the stars appear to be twinkling.

(v) Planets are closer to the earth and thus are extended sources of light. In the case of planets, the amount of light entering our eyes is significantly large. Even if the physical conditions of the earth’s atmosphere keep changing, the total variation in the amount of light entering our eyes is negligible. Therefore, the amount of light coming from the planets does not change with slight variations in atmospheric conditions. Therefore, planets do not twinkle.