The Mach-Zehnder interferometer is used to determine the phase shift caused by a small sample which is to be placed in one of two beams from a coherent light source.

There are - in contrast to the Michelson interferometer - two detectors A and B.

Function:

A coherent beam is split up by a beamsplitter and each is reflected by a mirror. The two beams pass a second beamsplitter and enter detector A and B, respectively.

There are some simple rules for phase shifts due to material (i.e. non-vacuum, which has a refractive index of exactly n=1):

- reflection/refraction at a surface with lower n causes NO phase shift

- at a surface with higher n there is a shift of half a wavelength

- the speed of light is slower in material with n>1. This means that in a slab of material the wavelength is decreased by its n:

<math>\lambda_{mat} = \frac{\lambda_{vac}}{n}<math>

Without a sample there is no phase difference (thus yielding constructive interference) for the two beams in detector A but complete destructive interference in the other one. If a sample in placed in a beam, there will be a variation in the intensities for A and B, which allows the calculation of the phase shift.