Light Rays and Normal Congruence

If you can form a bundle of rays, potentially pointing in various directions, that all happen to be orthogonal to a given wavefront, then these rays form a normal congruence.

The statement is then, "a group of rays will preserve its normal congruence after any number of reflections and refraction." This is an equivalent statement as, "rays will be orthogonal to wave fronts as long as the medium the wave is traveling in is isotropic.

Of course, if the media is anisotropic then the "rays" are no longer perpendicular to the wave fronts. That is to say, the direction of energy flow (the Poynting vector) may not be in a direction perpendicular to the wave fronts in an anisotropic medium.