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MENTAL MODEL · M0113

Coherence

Coherence
TechnicalHigh supportPhysics
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Version 1.0.0 · Updated 2026-07-28

CORE DEFINITION

In physics, coherence (also called phase coherence) states the possibility of interference between two waves. Two monochromatic beams from the same single light source always interfere with each other. If two wave sources have the same frequency and waveform, they are coherent. The physical sources of coherent waves are not strictly monochromatic; they may be partially coherent. However, beams from different physical sources are mutually incoherent. Coherence is an ideal intrinsic property of waves that enables stationary interference (interference at fixed time or space); that is, the property that waves must possess to produce significant interference phenomena. Coherence describes the characteristic that vibrations have a constant phase relationship at different positions at the same time, or at different times at the same position. It can be divided into spatial coherence, temporal coherence, and partial coherence. More generally, coherence describes the correlation properties between certain physical quantities of a single wave with itself, between multiple waves, and between wave packets.

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In physics, coherence (also called phase coherence) states the possibility of interference between two waves. Two monochromatic beams from the same single light source always interfere with each other. If two wave sources have the same frequency and waveform, they are coherent. The physical sources of coherent waves are not strictly monochromatic; they may be partially coherent. However, beams from different physical sources are mutually incoherent.

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The property of waves maintaining a fixed phase relationship in time and space is the basis of interference and holography.

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