Plane of incidence
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In describing reflection and refraction in optics, the plane of incidence (also called the meridional plane) is the plane which contains the surface normal and the propagation vector of the incoming radiation. 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 light also lies in the plane of incidence.
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.