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Interferometer, an instrument that uses the interference patterns formed by waves (usually light, radio, or sound waves) to measure certain characteristics of the waves themselves or of materials that reflect, refract, or transmit the waves. Interferometers can also be used to make precise measurements of distance. Interference patterns are produced when two identical series of waves are brought together.

Optical interferometers can be used as spectrometers for determining wavelengths of light and for studying fine details in the lines of a spectrum. Optical interferometers are also used in measuring lengths of objects in terms of wavelengths of light, providing great precision, and in checking the surfaces of lenses and mirrors for imperfections. In astronomy, optical interferometers make it possible to determine the diameter of large, relatively nearby stars and the separation of very close double stars. Radio interferometers are used in astronomy for mapping celestial sources of radio waves. Acoustic, or sound, interferometers are used for measuring the speed and absorption of sound waves in liquids and gases.

Albert A. Michelson invented the first interferometer in the 1880's.

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