Glass Slab Emergent Ray Direction Explained

When a ray of light passes through a rectangular glass slab, it undergoes refraction twice: once upon entering the slab and again upon exiting.

First Refraction: Light enters the glass slab from air. Since glass is optically denser than air, the light ray bends towards the normal.

Second Refraction: The light ray exits the slab from the parallel face back into the air. As it moves from a denser medium (glass) to a rarer medium (air), the light ray bends away from the normal.

Crucially, because the entry and exit faces of the rectangular slab are parallel, the angle of incidence at the first surface equals the angle of emergence at the second surface relative to the normal. This geometry ensures that the emergent ray is parallel to the original incident ray. The ray experiences a lateral displacement but maintains its original direction.

Therefore, the emergent ray is parallel to the incident ray.