light interference. In a standard Michelson
interferometer set up, a coherent light source (the
Laser) is split into two beams. One beam is used
as a reference while the other beam is reflected
back from a mirror or retro-reflector at some
distance. It is then merged with the reference
beam, producing interference. The interference
fringes are counted as the external path length
changes. Because the wavelength of the Laser is
known and is highly stable, the distance can be
calculated from the number of fringes.