We know about light and also know that light travels in a straight line path in a medium or two different mediums with same density.
Now a question arises what happens when light travels from one medium to another with different densities for example from air to glass.
When light ray is made to travel from one medium to another say from air to glass medium then light rays bend at the boundary between the two mediums.
So, the bending of light when it passes from one medium to another is called Refraction of light.
The refraction of light takes place on going from one medium to another because the speed of light is different in two media.
Medium in which speed of light is more is called optically rarer medium and medium in which speed of light is less is known as optically denser medium. For example glass is an optically denser medium than air and water.
NOTE:- When light goes from rarer medium to denser medium it bends towards the normal and when it goes from denser medium to rarer medium it bends away from the normal.
Refraction through a rectangular glass slab
To understand the refraction of light through a glass slab consider the figure given below which shows the refraction of light through a rectangular glass slab.
Here in this figure AO is the light ray travelling in air and incident on glass slab at point O.
Now on entering the glass medium this ray bends towards the normal NN’ that is light ray AO gets refracted on entering the glass medium.
After getting refracted this ray now travels through the glass slab and at point B it comes out of the glass slab as shown in the figure.
Since ray OB goes from glass medium to air it again gets refracted and bends away from normal N1N'1 and goes in direction BC.
Here AO is the incident ray and BC is the emergent ray and they both are parallel to each other and OB is the refracted ray.
Emergent ray is parallel to incident ray because the extent of bending of the ray of light at the opposite parallel faces which are PQ (air-glass interface) and SR (glass-air interface) of the rectangular glass slab is equal and opposite.
In the figure i is the angle of incidence, r is the angle of refraction and e is the angle of emergence.
Angle of incidence and angle of emergence are equal as emergent ray and incident ray are parallel to each other.
When a light ray is incident normally to the interface of two media then there is no bending of light ray and it goes straight through the medium.