If you dip a pencil in a bowl of water and observe the shape you will see that the pencil seems to be bent near the surface of water. This is due to a phenomenon known as refraction. Refraction of light is defined as the change in direction of a ray of light as it passes from one medium to another, i.e. air to glass or vice-versa. Refraction occurs because light travels at different speeds through transparent materials of different densities. For example, light moves at about 300,000 kilometers per second through air, but slows down in water, and much slower in glass.
Refraction of Light
When a ray of light passes form a rarer to a denser medium, it bends towards the normal and when it is the other way round, it bends away from the normal. The normal it defines as the perpendicular line to the interface of the two media. Also when a ray passes form one medium to another, its speed also changes. If the ray goes form a rarer to a denser medium its speed decreases and vice-versa.
When a ray of light passes fro, air into glass, it bends towards the normal. When it passes out into air again it bends away from the normal.
There are two basic laws of refraction.
The first law states that the ray that hits the surface called the incident ray and the ray that travels in second medium called the refracted ray and the normal all lie in the same plane.
The second law is called the Snails law which states that the ration of the sine of the angle of incidence to the sine of the angle of refraction is constant. The constant depends on the material through which the light passes. It is termed as refractive index.
The phenomenon of refraction explains many interesting facts. A coin in a bucket of water appears risen due to this phenomenon. The mirage in the deserts is also caused by refraction and total internal reflection of light. The twinkling of stars in caused due to he refracted light beams in the different layers of the atmospheric air.