What is the principle of reversibility ?

Hint:In physics, principle of reversibility is applied in optics for light rays.The term reversibility means any physical state of the body which can be reversed over time or follows the same path when traced backwards is called reversibility.

Complete answer:
Let us understand the principle of reversibility used in optics. The principle of reversibility of light states that” consider a ray of light which travels from point A to point B through a particular path say $C$ then if we trace the light ray in backward direction, it will follow the same path to reach point A from point B and follows the same path $C$ “This is called principle of reversibility of light.

Let us understand by taking a simple example: Consider ray of light falls from medium A on a refraction interface to medium B and its trace of path is shown in figure through path $C$ as:

The light has followed a particular path $C$ . Now, according to the principle of reversibility of light if light ray have to travel from medium B to medium A then it will follow the same path in opposite direction which can be represented as:

Hence, Light follows the principle of reversibility.

Note:It should be remembered that, light is a form of energy which follows the principle of reversibility but if we take sound waves which is also a form of energy but sound waves don’t follow the principle of reversibility hence, sound waves can change their path if they traced backwards.