The incident ray bends towards the nornal when travels from air to glassand bends away from the normal whentravels through glass to air. Thus, thelight emerges from a glass slab in a direction parallel with that in which it enters the glass slab.

Explanation:

When a ray of light passes through a rectangular glass slab, the incident ray and the emergent ray are said to be parallel. This phenomenon is known as refraction.

Refraction:

Refraction is the bending of light when it passes from one medium to another. The amount of bending depends on the angle at which the light enters the second medium and the difference in the refractive indices of the two media.

Incident Ray:

The incident ray is the ray of light that falls on the surface of the rectangular glass slab. When the incident ray enters the glass slab, it bends towards the normal because the refractive index of the glass slab is greater than the refractive index of the air.

Normal:

The normal is an imaginary line perpendicular to the surface of the rectangular glass slab at the point where the incident ray enters the glass slab.

Emergent Ray:

The emergent ray is the ray of light that emerges from the rectangular glass slab after refraction. The emergent ray is parallel to the incident ray because the angle of refraction is equal to the angle of incidence and the glass slab has two parallel surfaces.

Conclusion:

Thus, when a ray of light passes through a rectangular glass slab, the incident ray and the emergent ray are said to be parallel because the angle of refraction is equal to the angle of incidence and the glass slab has two parallel surfaces.