What is atmospheric refraction ? Explain briefly.

Explanation

Atmospheric refraction is the phenomenon of bending of light passing through the earth.s atmosphere.
The earth is surrounded by a layer of air called the atmosphere. The density of air in the atmosphere is not the same everywhere. The hotter air is lighter (less dense) than the cooler air (more dense) above it.
In general, density is highest at the earth.s surface and goes on decreasing as we move higher.
The refractive index of air depends on its density. The lower the density of air, the lower is its refractive index.
Thus, the upper layers of the earth.s atmosphere are optically rarer as compared to the lower layers.
Thus, light coming from the Sun and stars passes through a medium (air) of continuously increasing refractive index before it reaches the observer on the earth and hence its direction of propagation continuously changes.
Since the physical conditions of the refracting medium (air) are not stationary, the apparent position of an object as seen through the hot air fluctuates continuously.
This wavering is thus an effect of atmospheric refraction (refraction of light by earth.s atmosphere) on small scale in our local environment.
Some phenomena based on atmospheric refraction are :
(1) Twinkling of stars
(2) Advance sunrise, i.e., the sunrise is observed two minutes earlier than the actual sunrise.
(3) Delayed sunset, i.e., the sunset is observed two minutes later than the actual sunset.
(4) Stars seem higher than they actually are.
(5) The Sun appears oval (or flattened at the sunrise and the sunset, but appears circular at noon.