Am I correct in thinking that the act of observing which slit a "particle" goes through causes the wave function to collapse, ergo making the source of the "particle" we detect on the back screen to actually be on our detection side of the slits rather than the emitter side? I've drawn it, in case what I said didn't make sense... Now, nobody has ever taught me so this might be wrong: The way I understand optics, a collimator for example, doesn't actually redirect "particles" to travel in a straight line, what it actually does is reshape the probability that the wavefunction will collapse somewhere that is in that straight line path (I know they are the wrong words)... I should add, I understand the wavefunction does collapse at the optics and is re-emitted in it's new shape/configuration. Granted my understanding might be wrong; but this is the way I imagine it to be. Until the energy (photon) is absorbed (observed) it will continue to bounce (be emitted) from points in space (mirrors, lenses). Also I don't believe in wave-particle duality - everything is a wave until it is a "particle", there is no such thing as both and when something does behave like a particle (a proton say) it is really that the contributory wave sources (quarks) are quantum jumping to new points of origin, giving the impression that a particle exists. I don't think quarks are fundamental either, but that's another topic.