What is the reason that sunlight have short temporal coherence? I know everybody says that coherence length is inversely proportional to bandwidth. Does it mean that if sunlight passes through a very narrow bandpass filter the light will have infinite coherence length?
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$\begingroup$ Seems like there is 2 competing view: The coherence length will not increase since the dipole radiation is incoherent. The coherence length will increase because it is inversely related to bandwidth. Both are exactly what I think which is why I am confused... $\endgroup$– kmbCommented Mar 9, 2022 at 9:14
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2 Answers
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From this presentation:
. Temporal Coherence is a measure of the correlation of light wave’s phase at different points along the direction of propagation – it tells us how monochromatic a source is.
. Spatial Coherence is a measure of the correlation of a light wave’s phase at different points transverse to the direction of propagation - it tells us how uniform the phase of a wavefront is.
Italics mine
Sunlight is the product on the surface of the sun of innumerable radiations from excited atoms and moving charges.
By its generation there can be no long time coherence in sunlight, in the sequence of light arriving, since the sun is not an antenna radiating :).
The notes show how to generate coherence in the lab.
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Sunlight has short temporal coherence for two reasons. First, it is the sum of light waves of many different frequencies; secondly, even the waves of some given frequency come from a large number of separate oscillators (electrons) whose phase origins are randomly distributed.
If you pass any light at all through a sufficiently narrow filter then its coherence length will increase. So, yes, this is true for light that originated in the sun, just as for any other light.
answered Mar 8, 2022 at 11:10