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I have two identical point light sources A & B emitting the same frequency. They are equidistant from the single slit and from the line passing perpendicularly through the middle of the slit towards the screen. The light from the single slit is focused onto a screen a sufficient distance away for fraunhofer diffraction to occur. This seems like a similar setup should a double slit be placed before a single slit. But I am uncertain how this would work...

A) Would an interference pattern even occur? After all, the single slit would turn incoherent light into coherent light. And light arriving at different angles and thus phases would count as incoherent light despite having the same frequency and amplitude, right? In this case, I would get just the normal single slit diffraction pattern. Admittedly, I don't quite believe that this would happen...

B) I am relatively certain that covering one of the sources would result in the single slit profile, just shifted away from the midway line, away from the light source due to the path differences between the rays travelling from the source to either edge of the slit. Would then uncovering both sources just result in two partially overlapping single slit profiles, whose central maxima (0th order) would both be shifted away from the midline?

Cheers and thanks for reading.

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  • $\begingroup$ if they are not of same phase $i.e$ not from coherent sources, you'll not find the YDSE patterns exactly, but maybe some specific patters or even not, depending on intensity profiles. $\endgroup$ – Pritam Sarkar Sep 5 '20 at 18:12
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  1. your "the single slit would turn incoherent light into coherent light." is wrong.
  2. you get two interference patterns from the two sources , the may overlap, if the two sources are too near together
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  • $\begingroup$ Would I get two interference patterns despite only having a single slit? Why not two single slit profiles? $\endgroup$ – stoneskin Sep 5 '20 at 18:34
  • $\begingroup$ yes you would get two interference patterns, $\endgroup$ – trula Sep 5 '20 at 21:16

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