Interferometers are used both to measure distances in terms of wavelengths and to determine wavelengths of particular light sources. The interferometer configuration used by Michelson relies on an optical device called a beam splitter. The beam splitter directs approximately half of the light from an appropriate light source toward a fixed mirror and directs the other half toward a mirror mounted on a precise slide. The light that travels the path from the fixed mirror to the beam splitter is then recombined with the light reflected from the moving mirror. As the observer brings the light in and out of phase by moving the latter mirror with turns of a micrometer screw, he or she views through the instrument's telescope the light interfering with itself. The observer experiences this interference as an alternation of darkness with light. Each of these alternations --called fringes-- equals half of a wavelength. By the late Nineteenth Century, a scientist could measure wavelength by counting fringes against a length standard.
This interferometer was used by Irwin G. Priest for measuring the wavelength of neon light in 1912 --the first precision measurement of wavelengths published by NBS. It was designed by National Bureau of Standards (NBS) Director Stratton and was one of the first instruments built in the agency's machine shops. It appears that Dr. Stratton combined the system of mechanical motion peculiar to a Michelson-type interferometer with the Fabry-Perot arrangement of mirrors.
The beam splitter of the Michelson Interferometer is replaced with a partially silvered mirror in the Fabry-Perot, and the fixed mirror is removed. In this configuration the light wave interference occurs when the light reflected from the moving mirror back to the partially silvered surface is recombined with the light that was not transmitted through that surface initially. In this way fringes are produced in a manner similar to the Michelson interferometer.
After 1960, when the wavelength of light from a Krypton 86 lamp replaced the International Prototype Meter as the primary standard, the former relationship between artifacts of length and wavelengths of light was reversed, and the length of the meter bar was now determined by comparison to the wavelength standard. By counting the number of dark fringes observed as the interferometer's slide advanced the moving mirror, the wavelength of the light became the "scale" to determine how far the slide had moved. However, even before 1960, the interferometer was used in dimensional metrology labs. This was because the wavelength of light was a uniquely stable secondary standard. Moreover, unlike other instruments that could be termed "comparators," the interferometer enabled the comparison of this stable representative of the International Prototype Meter with length standards having shorter nominal lengths such as gage blocks.
Follow a path that traces the historical development of length comparators.
