Why do stars twinkle ?

solution:
Twinkling of stars can be seen on a clear night. This is due to atmospheric refraction of light coming from the stars (star light). As the star light enters into the earth’s atmosphere, atmospheric refraction takes place due to gradually changing refractive index of the air.

Since the physical conditions of the refracting medium (earth’s atmospheric) are not stationary, star light flux (luminous flux) entering the eye of an observer continuously fluctuates. This means luminous energy entering the eyes per second from the star increases and decreases with time. Thus, the star sometimes appear brighter and at some other time fainter, causing the ‘twinkling of stars’.