I have a question regarding Young's double slits experiment. To my understanding, wave function of a photon somehow collapses according to the probability function (which has a interference pattern). But at the very moment the wave hits the screen, it seems to me that there should be no interference. (just two peaks, also at the last moment) It would take a fraction of time (that the photon travels a few wave length distance) before the interference pattern emerges. So I wonder, the wave function might collapse even before the interference pattern shows up? at the very moment when the wave touches the screen? (oppose to collapsing only when sufficiently interfered) I think experiments can tell the difference; the double-slit pattern would enclose a small portion of two single-slit patterns, and that portion would vary according to the distance between the slits and the screen. My question is: 1. My understanding of wave function collapse is correct? Does one need a new interpretation than the wave function collapse? Or I got it all wrong? 2. How's the experimental result? Double slits only show purely interferential pattern? Did they look at the chance that it may contain single-slit patterns? Could somebody help me on this topic? Thank you.