Plane of incidence

In describing reflection and refraction in optics, the plane of incidence (also called the incidence plane or the meridional plane^{[citation needed]}) is the plane which contains the surface normal and the propagation vector of the incoming radiation.^[1] (In wave optics, the latter is the k-vector, or wavevector, of the incoming wave.)

When reflection is specular, as it is for a mirror or other shiny surface, the reflected ray also lies in the plane of incidence; when refraction also occurs, the refracted ray lies in the same plane. The condition of co-planarity among incident ray, surface normal, and reflected ray (refracted ray) is known as the first law of reflection (first law of refraction, respectively).^[2]