What is the principle of reversibility of light? Show that the incident ray of light is parallel to the emergent ray of light when light falls obliquely on a side of a rectangular glass slab.

The principle of reversibility of light states that, if the direction of a ray of light is reversed due to reflection or refraction, then it will retrace its path.

In the figure given below, a light ray travelling in the air, strike the rectangular glass slab, and gets refracted and bends towards the normal (as it is going from rarer to denser medium). Again, a change in the direction occurs when the refracted ray travelling in glass emerges into the air. Here, the light ray bends away from the normal (as it is going from denser to rarer medium). We can see that the incident ray and emergent rays are parallel to each other. These rays are parallel because the bending limits on the opposite and parallel faces of the slab are equal and opposite.