Interferometer

I

Introduction

Interferometer, instrument that utilizes the phenomenon of interference of light waves for the ultraprecise measurement of wavelengths of light itself, of small distances, and of certain optical phenomena (see Light). Because the instrument measures distances in terms of light waves, it permits the definition of the standard meter in terms of the wavelength of light (see Metric System).

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 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 caused by moving the mirror through a measured distance.

II

Uses

When the wavelength of the light used is known, small distances in the optical path can be measured by analyzing 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 retardation of the beam. The principle of the interferometer is also used to measure the diameter of large stars, such as Betelgeuse. Because modern interferometers can measure very tiny angles, they are further used—again, on such nearby giants as Betelgeuse—to gain images of actual 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.

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III

Michelson-Morley Experiment

Historically, the best-known interferometer is the one devised 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, erroneously presumed to exist as the carrier of light waves. Were the earth moving through a stationary ether, light traveling in a path parallel to the earth's direction of motion would take longer to pass through a given distance than light traveling the same distance in a path perpendicular to the earth's motion. The interferometer was arranged so that a beam of light was divided along two paths at right angles to each other; the rays were then reflected and recombined, producing interference fringes where the two beams met. If the hypothesis of the ether were correct, as the apparatus was rotated the two beams of light would interchange their roles (the one that traveled more rapidly in the first position would travel more slowly in the second position), and a shift of interference fringes would occur. Michelson and Morley failed to find such a shift, and later experiments confirmed this. Today the propagation of electromagnetic waves through empty space has replaced the concept of the ether.

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