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## Main Question or Discussion Point

Hi,

I just passed a course of a quantum coherence theory (or quantum optics). However, during the course i found more questions rather than answers.

One particular:

Imagine there's a white light coming from the sun. What is the coherence length of this light, according to the quantum coherence theory predictions (we know the temperature of the source, which is the only parameter of the light coming, so there should be a particular answer - the length in micrometers).

Specifically, imagine, we take a beam of the light coming from the sun and let it pass through a diffraction grating. Then we use a slit to get the light with a particular wavelength. And then we measure the coherence length of the light that passes through the slit. Again? What is the value of it and how it depends on the wavelength.

One more: The slit has of course a finite width, so the light that passes has some finite bandwidth. Let's measure the coherence length for one particular wavelength by changing the slit width, i.e. the bandwidth of the light? What would happen? Will the coherence length of this passed beam increase up to infinity, when the slit width will approach zero??

There is maybe a problem, that when the slit is very small, almost no photon passes through and we cannot even measure its coherence length. But anyway, if somebody has any idea about how it works, i'd really appreciate if he shed some light on this. Thanks

I just passed a course of a quantum coherence theory (or quantum optics). However, during the course i found more questions rather than answers.

One particular:

Imagine there's a white light coming from the sun. What is the coherence length of this light, according to the quantum coherence theory predictions (we know the temperature of the source, which is the only parameter of the light coming, so there should be a particular answer - the length in micrometers).

Specifically, imagine, we take a beam of the light coming from the sun and let it pass through a diffraction grating. Then we use a slit to get the light with a particular wavelength. And then we measure the coherence length of the light that passes through the slit. Again? What is the value of it and how it depends on the wavelength.

One more: The slit has of course a finite width, so the light that passes has some finite bandwidth. Let's measure the coherence length for one particular wavelength by changing the slit width, i.e. the bandwidth of the light? What would happen? Will the coherence length of this passed beam increase up to infinity, when the slit width will approach zero??

There is maybe a problem, that when the slit is very small, almost no photon passes through and we cannot even measure its coherence length. But anyway, if somebody has any idea about how it works, i'd really appreciate if he shed some light on this. Thanks