. Explain with help of a labelled diagram, the cause of twinkling of stars
The twinkling of a star is due to atmospheric refraction of star light. The atmospheric refraction occurs in a medium of gradually changing refractive index. Since the atmosphere bends point of star light towards the normal, the apparent position of the star is slightly different from the actual position. This apparent position of the star is not stationary, but keeps on changing slightly as the physical conditions of the earth's atmosphere are not stationary.
Since the stars are very distant, they act as point-sized sources of light As the path of rays of light coming from the star goes on varying slightly, the apparent position of the star fluctuates and the amount of light entering the eye flickers-- the star sometimes appears brighter and at other times appears fainter, which is the twinkling effect.