Principle of a radio interferometer.
An interferometer is any instrument that employs interference effects. Interferometers are used for measuring the wavelengths of light, radio waves, sound, or other wave phenomena; for measuring the refractive index (see refraction) of gases (Rayleigh interferometer); for measuring very small distances using radiation of known wavelength, or, in acoustics and radio astronomy (see below), 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 for the Michelson-Morley experiment. More accurate for wavelength measurements is the Fabry-Perot interferometer.
Interferometers in astronomy
Astronomical interferometers consist of linked telescopes which collect the electromagnetic radiation arriving from an astronomical object along two or more different paths and combines the results to form an interference pattern. The resulting resolution is much higher than that achievable by the component telescopes working independently.
Radio interferometers have been used widely in radio astronomy for many years; an example is the Very Large Array in New Mexico. More recently, optical interferometers have begun to play a major role in observational astronomy at visible wavelengths. The world's most powerful optical interferometer is the European Southern Observatory's Very Large Telescope. Space-based optical interferometers are also planned, including Darwin and the Terrestrial Planet Finder.