Interferometer

Interferometer, instrument that utilizes the phenomenon of interference of light waves for the ultra-precise measurement of wavelengths of light itself, of small distances, and of certain optical phenomena. Many forms of the instrument are used, but in each case two or more beams of light travel separate optical paths, determined by a system of mirrors and plates, and are finally united to form interference fringes. In one form of interferometer for measuring the wavelength (see also Wave Motion) of monochromatic light, the apparatus is so arranged that a mirror in the path of one of the beams of light can be moved forward through a small distance, which can be accurately measured, thus varying the optical path of the beam. Moving the mirror through a distance equal to one half of the wavelength of the light causes one complete cycle of changes in the pattern of interference fringes. The wavelength is calculated by measuring the number of cycles that occur as the mirror is moved through a measured distance.

When the wavelength of the light used is known, small distances in the optical path can be measured by analysing the interference patterns produced. This technique is used to measure the surface contours of telescope mirrors. The refractive indices of substances are also measured with the interferometer, the refractive index being calculated from the shift in interference fringes caused by the slowing down of the beam. The principle of the interferometer is also used to measure the diameter of large, relatively nearby stars, such as Betelgeuse. Because modern interferometers can measure very tiny angles, they are further used—again, on such nearby giant stars as Betelgeuse—to gain images of brightness variations on the surfaces of such stars. This technique is known as speckle interferometry.

The interferometer principle has also been extended to other wavelengths, and it is now widely employed in radio astronomy.

Historically, the best-known interferometer is the one devised in about 1887 by the American physicist Albert Michelson for an experiment he conducted with the American chemist Edward Morley. The experiment was designed to measure the absolute motion of the earth through a hypothetical substance called the ether, through which it was believed that light and electromagnetic waves in general were propagated. See also Michelson-Morley Experiment.

In this instrument a beam of light is partially reflected several times between two parallel, partially silvered, glass plates in such a way that the transmitted parts of the original beam interfere to give a series of sharp fringes. It has been used to investigate the fine structure of spectral lines, and also to measure wavelengths.