Coherence (physics)

Coherence measures how consistently waves maintain a predictable phase relationship across time or space. It determines whether wave amplitudes combine reproducibly to produce interference.

Mutual coherence function: A correlation function measuring how strongly the complex amplitudes of a wave at two events are related. It quantifies the phase correlation between the two times or locations that define coherence.
Phase (waves): The position of a point in a wave’s oscillation cycle, usually expressed as an angle. Coherence is defined by how phase differences behave across time or space.
Laser: A light source that amplifies radiation by stimulated emission and typically produces a narrow, coherent beam. Laser light’s strong coherence supports stable interference and precise optical control.
Incoherent light: Light whose phase fluctuates irregularly across time or space, limiting stable interference. It provides the direct opposite of strongly phase-correlated illumination.
Degree of coherence: A normalized measure of correlation between complex wave amplitudes, with magnitude from zero to one. Its magnitude expresses the strength of coherence independently of wave intensity.
Superposition principle: The rule that overlapping linear waves add their amplitudes. Coherence matters because superposed waves can reinforce or cancel according to their phase.
Holography: An imaging method that records and reconstructs a wavefront using interference. Recording a stable interference pattern requires coherent illumination.
Monochromaticity: The property of radiation having a single frequency or a very narrow frequency range. Narrow bandwidth often supports long temporal coherence, but frequency purity and coherence are distinct properties.
Temporal coherence: The consistency of a wave’s phase over time at a fixed location. It determines how long a wave remains phase-predictable at one point.
Autocorrelation: A measure of similarity between a signal and a shifted copy of itself. Temporal coherence is commonly expressed through correlations between a wave and its delayed copy.

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