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What would happen with a non-point but coherent source (some kind of ideal flashlight which produces perfectly coherent light for example)?

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  • $\begingroup$ In a perfect world, this would appear like a point source in infinite distance. If the source dimensions are not much bigger than the slit dimensions, however, we should treat this as an infinitely far point source, then a circular (or whatever) hole at the actual source, then the double slit (and finally the screen). In general, the result looks like the interference between lots of basic interference patterns, and these may cancel out the basic interference patterns by so much that they do not appear any more. (cont) $\endgroup$ – Hagen von Eitzen Apr 19 at 10:23
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    $\begingroup$ (cont) - Put differently, if we remove the double slit, we have the interperence patttern from a round hole; if we remove the round hole, we have the interference pattern from a double slit. With both obstacles present, we cannot expect that the result is always the pattern belonging to only one of them $\endgroup$ – Hagen von Eitzen Apr 19 at 10:27
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    $\begingroup$ Related Why must the single slit in a double slit experiment be narrow $\endgroup$ – Farcher Apr 19 at 12:21
  • $\begingroup$ I beleve before the invention of lasers, a point source was used in order to induce coherence from sources that were incoherent. $\endgroup$ – anna v Apr 19 at 16:27

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