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

Explanation

When a rectangular glass slab is immersed in any medium, the ray incident on the slab suffers two refractions, and emerges parallel to itself but displaced//shifted laterally as shown Fig. This is because deviation suffered by the ray in first refraction at one face of slab is equal and opposite to deviation suffered by the ray in 2nd refraction at the other face of the slab.