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Section 50.4 Interferometers

Interference of light has been used to build instruments called interferometers that permit us for measuring length, wavelength, and refractive index extremely precisely. You can classify these instruments into two types:

  1. Two-beam interferometer
  2. Multiple-beam interfermometer

In a two-beam interferometers, e.g. Michelson interferometer and Mach-Zehnder interferometer, a coherent source of light, such as the light from a laser beam, is split into two parts by an optical device such as a partially silvered mirror, which are brought together for interference after traveling different paths. In a multiple-beam interferometers, such as the Faby-Perot interferometer, a coherent source of light undergoes multiple partial reflections at the two sides of a medium. At each reflection some wave is transmitted through. All these transmitted waves are then focused together for collective interference.

Because of the sensitivity of interference, interferometers have many uses. You will see interferometers used for detecting microscopic variations on surfaces such as flatness and rougness in silicon wafers, to mapping the surface of Earth, analyzing air flows in wind tunnels, resolving spectral lines, etc. Perhaps, the most impressive uses of all is detection of gravitaitonal waves by Laser Interferometer Gravitational-Wave Observatory (LIGO). Visit LIGO website to learn more.

In the next two sections, we will go in detail for the working of two most-common interferometers - Michelson interferometer and Fabry-Perot interferometer.