Editing Coherence (physics) (section)

== Qualitative concept ==
[[File:Doubleslit.svg#/media/File:Doubleslit.svg|thumb|Two slits illuminated by one source show an interference pattern. The source is far to the left in the diagram, behind collimators that create a parallel beam. This combination ensures that a wave from the source strikes both slits at the same part of the wave cycle: the wave will have ''coherence''.]]
Coherence controls the visibility or contrast of interference patterns. For example, visibility of the [[double slit experiment]] pattern requires that both slits be illuminated by a coherent wave as illustrated in the figure. Large sources without [[collimation]] or sources that mix many different frequencies will have lower visibility.<ref>{{Cite book |last1=Born |first1=Max |title=Principles of optics: electromagnetic theory of propagation, interference and diffraction of light |last2=Wolf |first2=Emil |date=1993 |publisher=Pergamon Press |isbn=978-0-08-026481-3 |edition=6. ed., reprinted (with corrections) |location=Oxford}}</ref>{{rp|264}}

Coherence contains several distinct concepts. ''Spatial coherence'' describes the correlation (or predictable relationship) between waves at different points in space, either lateral or longitudinal.<ref>{{cite book|last1=Hecht|title=Optics|date=1998|publisher=Addison Wesley Longman|isbn=978-0-201-83887-9|pages=554–574|edition=3rd}}</ref> ''Temporal coherence'' describes the correlation between waves observed at different moments in time. Both are observed in the [[Michelson–Morley experiment]] and [[Young's interference experiment]]. Once the fringes are obtained in the [[Michelson interferometer]], when one of the mirrors is moved away gradually from the beam-splitter, the time for the beam to travel increases and the fringes become dull and finally disappear, showing temporal coherence. Similarly, in a [[double-slit experiment]], if the space between the two slits is increased, the coherence dies gradually and finally the fringes disappear, showing spatial coherence.  In both cases, the fringe amplitude slowly disappears, as the path difference increases past the coherence length.

Coherence was originally conceived in connection with [[Thomas Young (scientist)|Thomas Young]]'s [[double-slit experiment]] in [[optics]] but is now used in any field that involves waves, such as [[acoustics]], [[electrical engineering]], [[neuroscience]], and [[quantum mechanics]].  The property of coherence is the basis for commercial applications such as [[holography]], the [[Sagnac interferometer|Sagnac]] [[gyroscope]], radio [[Phased array|antenna arrays]], [[optical coherence tomography]] and telescope interferometers ([[Astronomical optical interferometer]]s and [[radio telescope]]s).