Atmospheric refraction: It is the deviation of light or other electromagnetic waves from a straight line as it passes through the atmosphere due to the variation in air density as a function of height.Refraction causes due to the velocity of light through air, decreasing with increasing density.This includes many interesting topics such as,
1) Twinkling of stars
The twinkling of stars is caused due to atmospheric refraction of starlight, the light of the stars entering the earth’s atmosphere undergoes refraction continuously before it reaches the earth.
The atmospheric refraction occurs in a medium of gradually changing the refractive index. Refractive index: A ray of light travels obliquely one transparent medium to another transparent medium and will change its direction in the second medium.
since the atmosphere bends starlight towards the normal the apparent position of the star is slightly different from its actual position. The stars appear slightly higher than its actual position when viewed near horizon. so, the stars are twinkle when looks. Look at the picture. Not only this, there is a big interesting question about twinkling. The question is that.
Why do stars twinkle and planets do not?
Unlike stars, planets don’t twinkle. Stars are so distant that they appear as pinpoints of light in the night sky, even when we viewed through a telescope. Because all the light is coming from a single point, its path is highly susceptible to atmospheric interference (i.e. their light is easily diffracted).
The much closer planets appear instead as tiny disks in the sky (a distinction more easily discerned with a telescope than with the naked eye). Their apparent sizes are usually larger than the pockets of air that would distort their light, so the diffractions cancel out and the effects of astronomical scintillation are negligible. Look at the above picture.