High Quality Content by WIKIPEDIA articles! The reflection coefficient is used in physics and electrical engineering when wave propagation in a medium containing discontinuities is considered. A reflection coefficient describes either the amplitude or the intensity of a reflected wave relative to an incident wave. The reflection coefficient is closely related to the transmission coefficient. Different specialties have different applications for the term. In optics, intensity and amplitude reflection coefficients are used. Typically, the former are represented by a capital R, while the latter are represented by a lower-case r.