What is the name of the law that states that the angle of incidence equals the angle of reflection?

The law that states the angle of incidence equals the angle of reflection is the law of reflection.

It applies when a ray strikes a reflecting surface such as a mirror. Both angles are measured from the normal, an imaginary line perpendicular to the surface at the point where the ray hits—not from the surface itself. The incident ray, reflected ray, and normal also lie in the same plane.

The principle is closely associated with geometric optics and can be derived from the wave nature of light using Huygens' principle. It works for smooth, specular surfaces, where rays reflect in an orderly way. Rough surfaces still obey the law at each tiny point, but their differently oriented surface normals scatter the reflected light in many directions.

A common mix-up is with Snell's law, which describes refraction as light passes between materials. Malus's law concerns the intensity of polarized light through a polarizer, while Brewster's law identifies the angle at which reflected light is completely polarized.