Also known as Fresnel reflection. That portion of the incident radiation that is reflected from the surface of a refractive material. It is a function of the refractive index of the material. At normal incidence:
where r is the fraction of the incident light to be reflected, and n is the refractive index of the material. If n = 1.5, for a light crown glass, r = 0.04, or 4 percent.