Interference between two beams of coherent radiation produces a spatial distribution of radiant energy that depends on the optical frequencies in the beams. Michelson's interferometer was created more than a century ago to test the existence of the "luminiferous ether" that supposedly permeated all space. In one of the greatest physics experiments of all time, Michelson failed to detect the presence of the ether. This result, while disappointing and puzzling to Michelson, was basic to Einstein's development of the theory of relativity.
Michelson's interferometer is today still of great importance as the basic component of the Fourier Transform Spectrometer. This device splits and recombines a beam of light such that the recombined beam produces a wavelength-dependent interference pattern. The basic components of the instrument are two perpendicular mirrors with a beam splitter between them. This is illustrated in the figure.
The incident energy is split into two beams that go to the two mirrors. These return the energy to the beam-splitter, where it is recombined. The combined wave emanating from the interferometer has an intensity that depends on the path difference between the two arms of the interferometer. With equal paths, constructive interference gives maximum intensity. If the light is monochromatic and the two arms have a half wavelength difference there is destructive interference, giving zero intensity. When one of the two mirrors is translated, all optical frequencies are converted into cosine waves of intensity. The superposition of all the cosine waves during the scan gives the complex time variation of intensity called the interferogram.