Editing Beam splitter (section)

==Phase shift==
[[File:Wavesplitter1.GIF|thumb|Phase shift through a beam splitter with a dielectric coating.]]
Beam splitters are sometimes used to recombine beams of light, as in a [[Mach–Zehnder interferometer]]. In this case there are two incoming beams, and potentially two outgoing beams. But the amplitudes of the two outgoing beams are the sums of the (complex) amplitudes calculated from each of the incoming beams, and it may result that one of the two outgoing beams has amplitude zero. In order for energy to be conserved (see next section), there must be a phase shift in at least one of the outgoing beams. For example (see red arrows in picture on the right), if a polarized light wave in air hits a [[dielectric ]] surface such as glass, and the electric field of the light wave is in the plane of the surface, then the reflected wave will have a phase shift of π, while the transmitted wave will not have a phase shift; the blue arrow does not pick up a phase-shift, because it is reflected from a medium with a lower refractive index. The behavior is dictated by the [[Fresnel equations]].<ref>{{citation |author1=Zetie, K P |author2=Adams, S F |author3=Tocknell, R M |title=How does a Mach–Zehnder interferometer work? |url=https://www.cs.princeton.edu/courses/archive/fall06/cos576/papers/zetie_et_al_mach_zehnder00.pdf |access-date=13 February 2014}}</ref>
This does not apply to partial reflection by conductive (metallic) coatings, where other phase shifts occur in all paths (reflected and transmitted). In any case, the details of the phase shifts depend on the type and geometry of the beam splitter.