Question

Why emergent ray is parallel to incident ray only in rectangular glass slab, Why not in triangular glass prism?

Answer 1

The emergent light is not parallel to the incident light in case of prism because the two surfaces of prism at which refraction occurs are not parallel unlike a glass slab and thus give a dispersion pattern when white light falls on the prism.

In a glass slab, both the normals are parallel to each other. This is because both the planes are parallel to each other. But in a rectangular prism, the surfaces are not parallel to each other. This causes the emergent ray NOT to be parallel to the incident ray.