In describing reflection and refractioninoptics, 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] (Inwave 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]
The orientation of the incident light's polarization with respect to the plane of incidence has an important effect on the strength of the reflection. P-polarized light is incident linearly polarized light with polarization direction lying in the plane of incidence. S-polarized light has polarization perpendicular to the plane of incidence. The s in s-polarized comes from the German word senkrecht, meaning perpendicular. The strength of reflection from a surface is determined by the Fresnel equations, which are different for s- and p-polarized light.