Question

what causes atmospheric refraction of light? mention its 2 effect

Answer 1

it happens due to the refraction of light in the atmosphere.

2effects:-
twinkling of stars,,,
sun can b seen 2minutes before  sunrise and 2 minutes after sunset 

Answer 2

The atmosphere of Earth is not evenly distributed, it is optically and molecularly denser at the bottom and and becomes rarer as we up, so the velocity of light changes from layer to layer. The molecules of different gases and dust partcles have different optical densities , moreover the hot air is optically rarer than cold air. These factors form differnt layers of air with different optical densities and thus cause the atmospheric refration.