Please,can someone explain me spatial coherence?
Spatial coherence is the spatial extent over which all the frequency components of a wave are in phase (or close to it) with each other.
As waves propagate they tend to lose their spatial coherence due to the phases of different frequency components drifting apart.
Spatial coherence is related to the size of a source, and means how well you can predict the field values *over there*, if I know what it is *here*. A point source produces a perfectly spatially coherent wavefront, while an infitely large source is perfectly spatially incoherent.
The van-Cittert-Zernike theorem is a very profound statement and worth spending time to understand. Briefly, the far-field diffraction pattern of a beam is proportional to the spatial coherence.
Spatial coherence is typically measured by a Younng's double-slit interfereometer, where the slit spacing is a measure of the spatial coherence.
Spatial coherence is the reason the holograms on credit cards look good outside in the sun, and terrible inside under fluorescent lights.
what does the SHORT longtudinal coherence do to its far-field diffraction when it passes through a pinhole? Thanks.
As I mentioned privately, since 'longitudinal' coherence refers to the waveband, the far-field diffraction pattern will appear smoothened out: each color will give a (for example) Airy pattern, but if there is a broad spectrum of colors, the total pattern will appear as a smooth blob.
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