Question

(a) What is atmospheric refraction? What causes atmospheric refraction?
(b) Why do stars twinkle on a clear night?
(c) Explain why, the planets do not twinkle at night

Answer 1

Hint : For this question, it is advised that the students should have a clear idea about why atmospheric refraction is actually caused, and what are the factors that can affect atmospheric refraction. They should also have a general idea about the relation between optical density of a medium and the extent of refraction.

Complete Step By Step Answer:
(a) Atmospheric refraction is defined as the refraction of light caused by the earth's atmosphere. Atmospheric Refraction occurs because the different layers of earth's atmosphere vary in terms of optical densities.
(b) Stars appear to twinkle at night because the light coming from a star undergoes atmospheric refraction. That happens because optical densities of air vary at different altitudes. The continuously changing atmosphere refracts the light from the star by different amounts each moment. The star-light reaching our eyes increases and decreases continuously and thus, the star appears to twinkle.
(c) Planets appear to be quite big. They can thus be considered as a collection of a very large number of point sources of light. There will be a dimming effect produced by some of the point sources, which will be nullified by brighter effects produced by some other point sources. Thus, the overall brightness changes negligibly and essentially remains the same. This is the reason why the planets do not appear to twinkle.

Note :
Atmospheric refraction actually occurs because the velocity of light becomes different when it passes through an optical medium denser than air. The velocity decreases due to the higher optical density, which results in deviation from the straight line path.