In 1881 Albert Michelson used an interferometer to successfully disprove the theory of a universal ether that existed till then. Later on, he determined the length of the basic meter in units of light wavelengths with this set-up. Still, the use of interferometers in performing technical length measurements only reached significance after the discovery of the laser as a coherent light source. Today, high precision length measuring instruments have become an important tool for many areas of the machine building industry.

The Michelson interferometer commonly uses one moveable mirror which is attached to the object for which a path length measurement on nm scale is performed. The Mach-Zehnder interferometer has no moving parts and the  operation is based on the retardation of one beam with respect to the other by changes of the index of refraction of the probe medium.

This experiment provides both, a Michelson and a Mach-Zehnder interferometer. The Mich­elson interferometer is used to demonstrate the classical interference patterns. For the Mach-Zehnder interferometer an evacuable tube is inserted into one of the beam paths and the interference pattern visualizes the changes of the index of refraction. The measurement of the index of refraction of air as a function of the pressure is made by using the vacuum pump.