Determination of the phase change on reflection from two-beam interference

Light reflected by a nondielectric material experiences a phase change on reflection that differs from light reflected by a dielectric material and other nondielectric materials. The complex degree of coherence for small optical path differences is derived for two-beam interference when the illumination source is extended, incoherent, and quasi-monochromatic. An analysis of the two-beam interference pattern reveals a simple relationship between the phase of the interference pattern at the point of maximum fringe visibility and the material-dependent phase change on reflection.