An interferometer is an instrument that allows a variety of precise measurements to be made through the interference patterns of light, sound, or radio waves. In the laboratory, the optical interferometer may be used to determine the thickness or refractive index of a material, and the acoustical interferometer can measure the velocity of sound in a gas or liquid. In optical astronomy the interferometer serves to determine the apparent diameter of stars. In radio astronomy the technique is used to obtain accurate measurements of the position of radio sources. The interferometer is also used in the study of stellar spectra. The principle of the interferometer, first demonstrated by A. A. Michelson in 1881, is based on the phenomenon that waves can intensify or extinguish one another. If a beam of parallel rays is directed perpendicularly onto a plane, all the waves will be in phase on arrival. If the incoming rays at two different spots of the plane are converged to a single point along paths of equal length, the waves will intensify one another at that point. If the plane is not perpendicular to the beam, a phase difference is introduced through the difference in time of arrival, and an interference pattern is created at the focal point. Information can be extracted from this interference pattern. In astronomy, for example, when the components of a double star are very close together, they cannot be separated with an ordinary telescope. Instead, two mirrors are used, spaced at a considerable distance apart and connected to a telescope. Each receives light from the star and reflects it toward the telescope, where it is finally assembled in a single focal point. Because the wavefront of the beam of one star makes a very small angle with that of the other, the two beams will not arrive simultaneously, and two interference patterns will be formed at the focus. By varying the distance between the mirrors, the intensity maxima of one image can be made to coincide with the minima of the other so that the interference pattern vanishes. This distance (D) is a function of the distance in arc (A) of the two stars and of the wavelength (Þ) of the light concerned.
When no interference pattern is observed, the size of angle a, in radians, becomes wavelength divided by 2 times distance (Þ/2D). This method was first used in 1920 to separate the components of Capella, which were found to be 0.045 seconds of arc apart. In the case of a single star, the two halves of the star's disk can be regarded as two half-disks whose centers constitute the centers of two light sources so that the star is seen artificially as double. When the interference vanishes, the diameter of the star is expressed by 1.22 Þ/D. Michelson first measured the diameters of the moons of Jupiter by this method in 1890. Modern interferometers can measure angles smaller than 0.001 second of arc. In addition, new techniques of interferometry are being applied to astronomy. Speckle interferometry, a technique that utilizes the pattern that emerges when light strikesÑor originates fromÑa rough surface, has been particularly useful in studying small interstellar objects such as planets and asteroids (see planetary systems). Because a star's surface also has irregular features that generate irregular light patterns, speckle interferometry reveals those features with more clarity than traditional methods. In Doppler imaging, a computer processes multiple images of the spectra of "hot" and "cool" regions on a rapidly rotating star. The computer is programmed to take into account the difference in light shift (see Doppler effect) when the regions are rotating toward or away from the measuring instrument. Combining the images produces a distinct picture of the various regions.