In refraction of light through a rectangular glass slab, the emergent ray is parallel to the direction of the incident ray-why?
This is because glass slab has parallel reflecting surfaces. The incident ray bends towards the normalinsidethe glass slab and the refracted ray bends away from the normal when it emerges from the glass slab. The extent of bending of the light ray at the two opposite parallel surfaces is equal and opposite. Hence the incident ray and the emergent ray are parallel to each other.