reflection law

(physics) When a wave, such as electromagnetic radiation or sound, is reflected from a surface in a sharply defined direction, the reflected and incident waves travel in directions that make the same angle with a perpendicular to the surface and lie in a common plane with it. Also known as law of reflection.

As applied to rays of light, sound, or radiant heat which strike a surface: the angle of reflection is equal to the angle of incidence, and the reflected and incident rays are in the same plane with a perpendicular to the surface.