define principle of reversibility
According to this principle, the path of a light ray is reversible.
In figure, a ray of light AO is incident at an angle i on a plane surface SS’ that separates two media 1 and 2.
The light ray is refracted along OB at an angle of refraction r.
Now if the refraction takes place from medium 2 to 1, the principle of reversibility requires that the ray of light incident along BO at O, at angle of incidence r in medium 2 will be refracted only along OA at an angle of refraction i in medium 1 and in no other direction than OA.