In its simplest interpretation the principle of reversibility is a corollary of __________ of the refraction of light.
Like with reflection, refraction also involves the angles that the incident ray and the refracted ray make with the normal to the surface at the point of refraction. Unlike reflection, refraction also depends on the media through which the light rays are traveling. This dependence is made explicit in Snell's Law via refractive indices, numbers which are constant for given media1.
Snell's Law is given in the following diagram.