Question
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.
Solution
The principle of reversibility of light states that the light ray after reflection or refraction, when reversed, will retrace its path.
Using the Snell's law,
When light ray falls from a rarer to denser medium, then
n21 = 
When retraced, the ray will move from denser to rarer medium.
That is,
Therefore,
