In numerous instances a ray propagating through an optical system penetrates a multilayer coating
such a bandpass filter, polarizer, beamdivider or antireflection coating. The additional retardance that the
coating adds to that ray is, to a first approximation, the optical thickness of the multilayer. Several
examples are given.
Philip Werner Baumeister,
"How an optical coating alters the path of a ray", Proc. SPIE 1270, Optical Thin Films and Applications, (1 August 1990); doi: 10.1117/12.20382; https://doi.org/10.1117/12.20382