Question

In a rectangular glass slab, the incident ray is parallel to the emergent ray, but not so in a triangular glass prism. Explain.

Answer 1

If we look at the the refraction through the glass prism diagram, it is evident that the beam of emergent light deviates back in the same direction with respect to the normal. The angle measure with respect to the normal are on the same side (unlike glass slab), so the emergent ray is not parallel to the incident ray.