The standard explanation for the interference pattern is one of constructive and destructive interference between the light waves from the two slits. But, am I right in thinking that light intensity is due to the number of photons in the wave at any position? i.e. in the light bands, there are the maximum number of photons hitting the screen and in the dark bands there are zero photons impacting the screen? In which case, although you can have constructive interference between light rays, you can never have destructive interference, since photons in one beam cannot be reduced to zero just because there are none in the other beam. If this is right, then I must deduce the dark bands are in fact the two wave troughs aligning and not a peak aligning with a trough. I would appreciate confirmation or otherwise please.

  • $\begingroup$ If two troughs align then it's still constructive interference. They will become peaks within some amount of time. And then troughs again. And on and on. Also keep in mind that EM waves don't look like waves on a string. $\endgroup$ Nov 30, 2018 at 13:37
  • $\begingroup$ Have a look at double slit experiments with single electron. physicsworld.com/a/… $\endgroup$ Nov 30, 2018 at 15:02

1 Answer 1


In the photon picture the individual photons don't interact with each other, so you can't talk about interference of different photon beams. Rather the wave function of each individual photon interferes with itself (that is after it passes the slit) in the same manner as a classical electromagnetic wave would. The intensity pattern then matches the probability of the photon hitting the screen at any particular spot, where places of high intensity align with a greater number of photons hitting that spot due to a greater probability of getting there in the first place. So your conclusion is wrong based on a wrong premise about how the photons interact.


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