Large sources without collimation or sources that mix many different frequencies will have lower visibility. For example, visibility of the double slit experiment pattern requires that both slits be illuminated by a coherent wave as illustrated in the figure. 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.Ĭoherence controls the visibility or contrast of interference patterns. The source is far to the left in the diagram, behind collimators that create a parallel beam. Qualitative concept Two slits illuminated by one source show an interference pattern. More generally, coherence describes the statistical similarity of a field (electromagnetic field, quantum wave packet etc.) at two points in space or time. 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. Two waves with constant relative phase will be coherent. Constructive or destructive interference are limit cases, and two waves always interfere, even if the result of the addition is complicated or not remarkable. 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 : 256 (destructive interference), depending on their relative phase. Beams from different sources are mutually incoherent. : 256 Physical sources are not strictly monochromatic: they may be partly coherent. Two monochromatic beams from a single source always interfere. In physics, coherence expresses the potential for two waves to interfere.
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