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?
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.
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.
