The ratio of the amplitude of the reflected electric field to the amplitude of the field incident at the optically smooth planar interface between two optically homogeneous media. The incident and reflected fields are plane harmonic and the interface is large in lateral extent compared with the wavelength of the illumination. May also be used for the ratio of reflected to incident irradiances, that is, the ratio of the normal (to the interface) component of the reflected Poynting vector to that of the incident normal. A better term for this quantity is reflectivity. The reflection coefficient (and hence reflectivity) depends on the angle of incidence of the illumination, its wavelength (by way of the wavelength-dependence of the relative refractive index of the two media), and its state of polarization. These coefficients taken together are sometimes called the “Fresnel formulae” or “Fresnel relations. ” Reflection coefficient may mean the ratio of any reflected to incident irradiance (reflectivity). See also transmission coefficient.