If you pass light through a double slit and back through the same slit again (say by using mirrors to bounce it back around) is there an interference pattern or particle pattern?

I'm curious what happens in this case? Will this create an interference pattern? A particle pattern? Will it make a more complex interference pattern?


1 Answer 1


You would get an interference pattern.

The only question is whether returning the light back collapses the wavefunction. Because a mirror does not collapse the wavefunction of incident light, then the result will be that the light passing through the slits (and back again) will produce an interference pattern.

The final resulting interference patter will obviously be different than light passing only one boundary.

  • $\begingroup$ Actually, there is not a good distinction between "interference pattern" and "particle pattern". Every interference pattern is composed of a pattern of "particle detections". But I think the OP is asking if the detected pattern will have interference fringes, and the answer is "yes", as you've said. $\endgroup$
    – S. McGrew
    Apr 9, 2019 at 0:21
  • $\begingroup$ Since you have lost information on the original wave function you certainly don’t get time reversal, that’s for sure. $\endgroup$
    – Jon Custer
    Apr 9, 2019 at 0:45
  • 1
    $\begingroup$ there is a series of MIT videos you might like to see, here is youtube.com/watch?v=J4Ecq7hIzYU $\endgroup$
    – anna v
    Apr 9, 2019 at 5:56
  • $\begingroup$ I appreciate the answer, and video link Anna and Chris. This gives me some things to think about. $\endgroup$ May 4, 2019 at 7:03

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