[croo]

First observations of quantum physics was that if you light through two slices of holes in a paper it makes more than two marks on the wall behind it - this proves that the light is some kind of a waveform or could act like one. As far as I know the fun starts when you try the same experiment with exactly one photon emitted. The single photon will simultaneously go through the two hole. This caused some confusion and we now call this phenomenon and related things "quantum". But thid is just my understanding based on a book, other commenters seems to be much more knowledgeable about the topic.

[int_19h]

The single photon registers in a single spot on the screen, as a particle should, and so there's no problem with assuming that it went through one hole or the other.

The fun starts when you start sending single photons through the holes one by one - again, every photon registers in a specific spot; but when taken in aggregate, those spots form the same interference pattern as with multiple photons.