Why does a light ray incident on a rectangular glass slab immersed in any medium emerges parallel to itself ? Explain using a diagram.

Explanation

In the given figure, EO is the incident ray, OO' is the refracted ray and O'H is the emergent ray.

The extent of bending of the ray of light at the opposite parallel faces AB (air-glass interface) and CD (glass-air interface) of the rectangular glass slab is equal and opposite. This is why the ray emerges parallel to the incident ray on a rectangular glass slab. However, the light ray is shifted sideward slightly. When glass slab in immersed in any medium the interface AB (medium-glass) and CD glass medium are equal and apposite so, the emergent ray will always be parallel to the incident ray.