Interferometer, << `IHN` tuhr fuh ROM uh tuhr, >> is a device that uses light waves or other waves to make precise measurements. Interferometers take advantage of a behavior of waves called interference. Interference occurs when two waves travel through the same space. The waves combine to create patterns of varying intensity. Scientists can analyze these patterns to make accurate measurements. Most interferometers use visible light, infrared, or radio waves. Others employ sound waves. Instruments known as atom interferometers use waves of matter. They make use of the fact that atoms, often thought of as particles, also have properties of waves.
A simple light interferometer splits a beam of light into two beams and brings them back together. The recombined beam shines on a screen or other device, forming a pattern of dark and light bands called interference fringes. Minor differences in the two paths the light traveled can create noticeable changes in the pattern of interference fringes. By studying the pattern, scientists can measure differences in the lengths of the two paths or the density or other properties of the materials through which the light traveled. They can also detect tiny imperfections in a surface and weak vibrations.
A device called a stellar interferometer can measure the positions of stars and other cosmic objects that give off light or other kinds of radiation. A typical stellar interferometer uses two telescopes. The telescopes collect light from the same star. Light from both telescopes travels through a series of mirrors. The mirrors recombine the light to form interference fringes.
A light wave traveling from the star will strike the nearer of the two telescopes sooner. The extra distance a wave must travel to reach the farther telescope is called the geometric delay. The path that light collected by the nearer telescope travels before the beams are recombined is called the internal delay. A set of adjustable mirrors allows astronomers to change the length of the internal delay. Interference fringes appear strongest when the internal delay equals the geometric delay. Scientists can then measure the internal delay to calculate the star’s position in the sky.
The pattern of interference fringes also varies with the size of the object being observed. Astronomers can therefore use interferometers to measure the diameters of stars and other heavenly bodies. A stellar interferometer gives much greater resolution than its individual telescopes. Resolution is the ability to distinguish among objects near one another. The resolution improves as the distance between the telescopes increases.
Some stellar interferometers combine large telescopes to produce sharp images of cosmic objects and to search for planets outside our solar system. These include the European Southern Observatory Very Large Telescope interferometer in Chile and the Keck Interferometer in Hawaii.
The French scientist Armand H. L. Fizeau developed the idea of a stellar interferometer in the mid-1800’s. In 1887, the American physicists Albert A. Michelson and Edward Morley used an interferometer to perform experiments that measured the speed of light and proved that it is constant in all directions. In 1920, Michelson and the American astronomer Francis G. Pease were the first to use an interferometer to measure the diameter of a star other than the sun.
