coherence time

In physics, coherence describes a fixed relationship between the phases of waves in a beam of radiation of a single frequency. When two beams of light are coherent, the phase difference between their waves remains constant. This is in contrast to noncoherent beams, where the phase relationship is random or changes over time.1

Coherent sources are necessary for stable interference patterns. Lasers are an example of a source that produces a coherent beam, where all components maintain a fixed relationship with each other, unlike incandescent sources.1

The concept of coherence is further explored through the mutual coherence function, which is crucial in understanding partially coherent light. This function relates to the cross-correlation of light at different points and time delays. For quasimonochromatic light, a coherence time ((\Delta t)) can be defined as (1/\Delta \nu), where (\Delta \nu) is the frequency bandwidth. A related coherence length ((\Delta l)) is given by (c/\Delta \nu), where (c) is the velocity of light. Interference occurs when path differences are less than this coherence length.2