Answer:
The refraction of light by the earth's
atmosphere is due to the gradually changing refractive index of air layers. The
physical conditions of the refracting medium (air) are not stationary and the
apparent position of the object as seen through the hot air fluctuates. This wavering
is thus an effect of atmospheric refraction on a small scale in our local environment.
Since, the atmosphere bends light from
the astronomical bodies viz, stars towards the normal, the apparent position of
the astronomical bodies is slightly different from its actual position. The
astronomical bodies appear slightly higher (above) than its actual position when
viewed near the horizon.
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.
The path of rays of light coming from the distant star goes on varying
slightly, the apparent position of the star fluctuate and the amount of
starlight entering the eye flickers that the star sometimes appears brighter
and at some other time fainter which gives us the twinkling effect.
The planets are much closer to the earth
and are thus seen as extended sources where as stars are very distant, the
approximate point-sized sources of light. If we consider a plane as a
collection of a large number of point-sized sources of light, the total
variation in the amount of light entering our eye from all the individual
point-sized sources will average out to zero, thereby nullifying the twinkling
effect. Hence, the planets do not twinkle.
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