In the previous chapter we have seen how light gets reflected when it is incident on a surface. Now let us see what happens when a ray of light travelling in one medium enters another medium of different optical density.
It is a matter of common experience that a spoon placed in a glass of water looks bent at the surface of water when viewed obliquely. This observation suggests that light changes its path as it passes from one medium to another. This change in the path of light is due to the fact that the velocity of light varies as it travels from one medium to another. The deviation in the path of light when it passes from one medium to another medium of different density is called refraction.
The twinkling of stars is due to atmospheric refraction of starlight. Since light bends towards the normal the apparent position of the star is slightly different from its actual position as it passes through the atmosphere. Hence the star appears slightly higher than its actual position. Due to changing condition of earth's atmosphere the apparent position of the star changes slightly and the intensity of light reaching the eye also fluctuates. This gives rise to the twinkling effect of the star.