Plane of incidence

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.

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.

The plane of incidence is defined by the incoming radiation's propagation vector and the normal vector of the surface.