Refraction: The phenomenon of bending of rays of light as they pass from one medium into another is called refraction of light.
Let a ray AO goes from optically rarer medium to an optically denser medium Fig. It is found that most of light passes through the denser medium but it bends towards the normal along OB. This bending of light is called refraction.
∠AON = i, which the incident ray makes with the normal is called angle of incidence.
∠BON' = r, which refracted ray makes with the normal is called angle of refraction.
Water is called optically denser medium and air is called optically rarer medium.
As ray of light goes from optically rarer medium to optically denser medium, it bends towards the normal and when the ray goes from denser medium to rarer medium, it bends away from the normal.
Illustrative Examples
1. Atmospheric Refraction: Rays of light coming from the sun through space bend very slightly towards the normal. The density of air is higher near the surface of the earth and gradually becomes less higher up. The sun rays, therefore, follow a curved path as they enter the atmosphere. The angle through which the rays actually bend is not more than 2°, but this is greater than the sun’s angular diameter so that the sun appears higher on the horizon than it really is. The sun, therefore, appears to set several minutes after it has actually sunk below the horizon, and can be seen a few minutes before it actually rises above the horizon. Fig.
2. Twinkling of Stars: Unless a star is directly above and the rays from it are normal to the atmosphere, atmospheric refraction causes an angular displacement of nearly 1/2° and a star is not where it appears to be Fig. This effect has to be taken into account by astronomers and navigators.
The atmosphere consists of several moving masses of air, some hot and some cold, some dense and some rarer. When light from a distant star enters the Earth's atmosphere, it is bent as it passes from one mass of air to another. The masses themselves are in motion. This gives rise to the twinkle in stars.
