Question

Why is the angle of emergence equal to the angle of incidence in a rectangular glass slab but not in a triangular glass prism?

Answer 1

When the incoming light beam hits the first air-glass interface of the glass slab or prism, the various colours get refracted at slightly different angles, or dispersion occurs.

This dispersed beam going through the glass when hitting the second glass-air interface gets refracted again.

In the case of a glass prism, the beam gets further dispersed.

In the case of the glass slab, which can be imagined as a glass prism with two sides parallel, the dispersion because of the first interface gets cancelled out and the light beam exits undispersed. This is because the emergent angle is same as the incident angle in the case of a glass slab.