Light is a collection of wavelengths that are fitted to a scale called the electromagnetic spectrum. I shall use visible light to talk about here, which is found in the center of the electromagnetic spectrum. The visible light colors are red, orange, yellow, green, blue, indigo, and violet. All these colors have specific wavelengths and frequencies. When you speak of reversibility of light, it has the capability of reflecting off an object. Sometimes, the light will travel back the same exact path it took to get to the object it reflected off of in the first place. The light wave simply reverses itself. You can demonstrate this by holding a flashlight in front of a plane mirror, shining it straight into the mirror and seeing the light being reflected back to the flashlight itself. Be sure to use a plane mirror (one that is flat) and not a concave or a convex mirror.
when light is incident on the surface of separation of two media,it gets deviated from its original path. Let us take an example to understand this better. If a ray of light travels from medium 1 to medium 2,it has a certain angle of incidence and angle of refraction,say 60 and 45 degrees. Now if you reverse the direction of the ray of light,such that it now travels from medium 2 to medium 1,THEN THE RAY OF LIGHT WILL GET REFRACTED IN MOVE IN THE SAME PATH,such that its angle of refraction and angle of incidence gets reversed. Thus in the above example,if we reverse the direction of the ray of light,the angle of incidence becomes 45 and the angle of refraction becomes 60.This is the principle of reversibility of light.
The principle that if a beam of light is reflected back on itself, it will traverse the same path or paths as it did before reversal.
The principle of reversibility states that light will follow exactly the same path if its direction of travel is reversed.
Hope that helps man!