A ray of light incident on a rectangular glass slab immersed in any medium emerges parallel to itself. Draw a labelled ray diagram to justify the statement.

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 the glass slab at point O. When the ray enters the glass medium it bends towards the normal NN′ that is light ray AO gets refracted on entering the glass medium. When the 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 the glass medium to air it again gets refracted and bends away from normal N₁N₁′ 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 the 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 the light ray and it goes straight through the medium.