I am reading a scientific essay about a experiment with interferometer. They use so called 50/50 beam splitter and perfect reflector. Well they have the input wave be the plane wave [tex]exp(ikx)[/tex], what interesting is: whenever the wave reflected by the splitter or perfect reflector, it add half of the wavelength to the wave, but if the wave was transmitted through the splitter, no wavelength was added to it. It is so confusing. What's the rule behind this? 1) When will the half wavelength being added to the wave? Only happen in reflection? 2) If the reflector is not perfect and the splitter is not 50/50 (30/70 or 80/20), will it still be a half-wavelength added? or any other portion of wavelength being added?