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I would like to know if the following are true:

Each time you fire a single photon, only one dot appears on the back screen.

Is there ever a case that while we are watching the experiment, that a single photon is fired but there is no new dot. And if so, is there a way to know whether it was because of interference or because the photon did not go through any slit?

user95890
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  • Related: http://physics.stackexchange.com/q/70855/2451 , http://physics.stackexchange.com/q/76162/2451 and links therein. – Qmechanic Oct 18 '15 at 14:32
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    If there is no dot this means that either the particle got lost on the way ("die not go through any slit", i.e. got absorbed/reflected by the barrier) or the detector failed to account for the particle (or because the particle was absorbed somewhere else behind the slits). The total particle flux is not reduced by the interference itself, so interference cannot make photons/protons vanish, the probability is only redistributed. – Sebastian Riese Oct 18 '15 at 14:34

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Each time you fire a single photon, at most one dot appears on the back screen.

Suppose the slits are cut from a mask. Suppose the area hit by the beam is 95% mask and 5% slit. Then you might expect a dot on the back screen 5% of the time.

mmesser314
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Yes one single photon shows up, assuming on screen or detector it is absorbed by an electronic layer. This is quantisation. ;-)

One photon appears... assuming 100% efficiency of the detector or screen :-) And as you pointed out, he could also not go through the slit at all. Or be absorbed/diffused by air (low probability). Or be specularly reflected on the screen (low probability since screens try not to be shiny).

Interference: this is the only no possible cause for not receiving a photon. Interferences just mean than some screen location are low or zero probability, so the photon will just be absorbed elsewhere.

Fabrice NEYRET
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