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How do interferometers measure very small distances or wavelengths of light?

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Optical interferometers measure small distances and wavelengths by exploiting the wave nature of light through a process called interference.

These instruments function by splitting a beam of light into multiple paths that travel different distances before being reunited. When the beams recombine, they undergo constructive or destructive interference, creating a pattern of bright and dark bands known as interference fringes.

In a Michelson interferometer, a beam is divided by a half-silvered mirror toward a fixed mirror and a movable mirror. As the movable mirror shifts, the path length difference changes, causing the interference fringes to shift. By counting these fringe displacements, scientists can determine the distance of movement with extreme precision—sometimes detecting changes as small as $10^{- 17}$ cm, as seen in the Laser Interferometer Gravitational-Wave Observatory (LIGO).12

Similarly, interferometers can determine wavelengths by analyzing these fringe patterns, as the spacing and intensity of the bands are directly related to the wavelength of the light used.13