coherence_physics

Coherence (physics)

Snippet from Wikipedia: Coherence (physics): Coherence expresses the potential for two waves to interfere. Two monochromatic beams from a single source always interfere.^: 286 Wave sources are not strictly monochromatic: they may be partly coherent. Beams from different sources are mutually incoherent.
When interfering, two waves add together to create a wave of greater amplitude than either one (constructive interference) or subtract from each other to create a wave of minima which may be zero^: 286 (destructive interference), depending on their relative phase. Constructive or destructive interference are limit cases, and two waves always interfere, even if the result of the addition is complicated or not remarkable.
Two waves with constant relative phase will be coherent. The amount of coherence can readily be measured by the interference visibility, which looks at the size of the interference fringes relative to the input waves (as the phase offset is varied); a precise mathematical definition of the degree of coherence is given by means of correlation functions. More broadly, coherence describes the statistical similarity of a field, such as an electromagnetic field or quantum wave packet, at different points in space or time.

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