Look at figure 1, here two media, air and glass are shown. Coming from air medium following the path AB, light ray incidents at point B of the separation surface PQ of two media. If the light ray goes straight it will follow the path BC’ but it changes its path and follows BC. This phenomenon of bending of my of light is called refraction. Therefore, when the ray of light travels obliquely from one transparent medium to another, the bending of ray of light at the surface of separation of the two media is called the refraction of light.
Fig (1) Fig (2)
In figure 2, the incident ray is AB, the refracted my is BC and NBNˊ is the normal drawn at point B on PQ. ˄ABN is called the angle of incidence i and ˄NˊBC is called the angle of refraction r.
Since the velocity of light differs from medium to medium, during the change of medium refraction takes place. When ray of light enters from a rarer medium (air) to a denser meditun (glass) the refracted ray bends towards the normal i.e. in this case i > r again when it enters from a denser medium to a rarer medium it goes away from the normal i.e. in this case r > i.