Thanks for this explanation. I've tried to read it some time ago but have not really coped with it. Now after reading again it was interesting for me to check if this is explained on some other internet sources and how exactly. So I checked one of the first top search results and here is what I saw: http://physics.stackexchange.com/questions/91695/double-slit-expirement-fundamentals-half-silvered-mirror-version

There some guy asked for an explanation of this experiment and answers are all about optical refraction and phase shifting, which honestly speaking would not clarify the matters for me. Now this is all ok about theory and numbers. But I tried to google some videos which will show this in action and found that actually it is not really easy (or not possible at all) to set up all these pieces and see it in action. For example, in this video https://www.youtube.com/watch?v=M6y_igUpyCg guys tries to set it up and does not succeed. Is that about he is usint a laser which in fact transmitts a bit different photons every time? Could you recommend some video where this experiment can be seen in action?