Rayleigh interferometer
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In optics, a Rayleigh interferometer is a type of interferometer which employs two beams of light from a single source. The two beams are recombined after traversing two optical paths, and the interference pattern after recombination allows the determination of the difference in path lengths.[1]

Light from a source (left) is collimated by a lens and split into two beams using slits. The beams are sent through two different paths and pass through compensating plates. They are brought to a focus by a second lens (bottom) where an interference pattern is observed to determine the optical path difference in terms of wavelengths of the light.