Question

why a ray emerges parallel to the incident ray

Answer 1

Since the refracting surfaces are parallel to each othe the emergent rays are parallel.

The angle of incidence on the first surface { from air to glass} be i and the angle of refraction at the first surface be r.

Since the surfaces are parallel the ray inside the glass falls at the other surface at angle r and therefore the ray emerges at angle i that is the angle of incidence.

Therefore the incident ray is parallel to the emergent ra