When light reflects (by Fresnel reflection) at an angle from an interface between two transparent materials, the reflectivity is different for light polarized in the plane of incidence and light polarized perpendicular to it. Light polarized in the plane is said to be p -polarized, while that polarized perpendicular to it is s -polarized. At a special angle known as Brewster's angle, no p -polarized light is reflected from the surface, thus all reflected light must be s -polarized, with an electric field perpendicular to the plane of incidence.