The complex reflection coefficients for the parallel (p) and perpendicular (s) polarizations of light that are normally incident upon an isotropic surface are proved to be stationary with respect to small changes of the angle of incidence in the neighborhood of zero. This is true not only for a single interface between isotropic media but also for any one-dimensionally inhomogeneous or multilayer reflecting structure that is stratified in the direction of the surface normal. For incident light of certain intensity, phase, and polarization, the intensity, phase, and polarization of the reflected light all remain stationary with respect to small near-normal angle-of-incidence variations to first order. A second-order analysis is carried out to determine the parabolic (quadratic) variation of various reflection parameters of an interface with an angle near normal incidence.
Journal of the Optical Society of America (1917-1983)
R. M. A. Azzam, "Stationary property of normal-incidence reflection from isotropic surfaces," J. Opt. Soc. Am. 72, 1187-1189 (1982)