Question

what causes atmospheric refraction of light? mention its two effects

Answer 1

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.

Two phenomenon occuring due to atmosperic refraction are-

Twinkiling of stars 
Rsising of sun 2 min. before sunrise and setting after 2 min. of 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.