rs 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 usedagain, on such nearby giants as Betelgeuseto 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.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 propagati...
