In the Michelson interferometer, a light beam is divided into
two beams of equal intensity which combine together again
before leaving the interferometer. The path difference of the
two beams can be changed by integer multiples of the wavelength,
with simultaneous observation of the interference
patterns which it generates.
The change in the path length can be read off from the interferometer,
and allows the determination of wavelength
when monochromatic light is used.
When spectrum lamps are used as light source, and with
appropriate filters, the interferometer can also be used to
determine coherence lengths.
In addition, the refractive index of suitable liquids, of air and
of other gases can be determined.