Interferometer, any instrument of measurement employing interference effects of waves. Interferometers are used for measuring the wavelengths of light, radio, sound, or other wave phenomena or the refractive index of gases. Some interferometers measure very small distances using radiation of known wavelength. In acoustics and radio astronomy, they are used for determining the direction of an energy source. In most interferometers the beam of incoming radiation is divided in two, led along paths of different but accurately adjustable lengths, and then recombined to give an interference pattern. Perhaps the best-known optical instrument is the Michelson interferometer devised in 1881. More accurate for wavelength measurements is the Fabry-Perot interferometer, in which the radiation is recombined after multiple partial reflections between parallel, lightly silvered glass plates.
See also: Interference.