For understanding the refraction of light through a glass slab let us consider the above figure consider the figure,
Here AO is the light ray travelling in air and incident on glass slab at point O.
When ray enters the glass medium it bends towards the normal
NN’ that is light ray AO gets refracted on entering the glass medium.
When ray gets refracted it now travels through the glass slab and at point B it comes out of the glass slab.
As 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 both are parallel to each other and OB is the refracted ray.
Emergent ray BC is parallel to incident ray AO because the extent of bending of the ray of light at the opposite parallel faces which are PQ and SR 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.