Why is the emergent ray parallel to direction of the incident ray in a rectangular glass slab?

In a rectangular glass slab, the emergent ray is parallel to the incident ray because of the principle of refraction. When light enters a denser medium (like glass) from a rarer medium (like air), it bends towards the normal. As the light exits the glass slab and reenters air, it bends away from the normal. The combination of these two refractions results in the emergent ray being parallel to the incident ray.