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coherent light

(beam of light at a single frequency)

Coherent light is light with both a constant phase difference and frequency, i.e., consisting of coherent waves. Lasers (the visible light analog of masers) produce such light.

One 1995 astronomical observation has been interpreted as a natural laser, but masers are more common. Lasers are also used for adaptive optics, in detectors for gravitational wave, and some high energy electromagnetic radiation, and in carrying out particle physics experiments of cosmological significance.