Reflexionsgesetz

The Reflexionsgesetz, or law of reflection, is a principle in geometrical optics. When a light ray encounters a smooth reflective boundary between two media, the ray is reflected such that the angle of incidence equals the angle of reflection. Both angles are measured with respect to the normal to the surface at the point of contact.

This law applies to all kinds of waves in appropriate linear media and follows from the boundary conditions for electromagnetic fields at interfaces. On rough or uneven surfaces, each microscopic facet obeys the law, so the macroscopic result is often diffuse reflection rather than a single reflected ray. The law is thus exact for specular reflection but explains only the directional behavior of the reflected light in real-world surfaces by considering surface roughness.

In optics, the law of reflection is often used together with Snell’s law (the law of refraction) to describe what happens at interfaces. The relative amounts of reflected and transmitted light are given by the Fresnel equations, which depend on the incidence angle, polarization, and the refractive indices of the media.

Applications include plane mirrors, periscopes, telescopes, and laser resonators. For curved mirrors, the law holds locally at each point on the surface, and standard ray-tracing or mirror equations describe image formation. While the law determines the direction of the reflected ray, the intensity of reflection is governed by material properties and wavelength.