In Feynman's lectures he describes partial reflection as occuring because the "little arrows" cancel out when the glass is certain thickness. I realize that there isn't an actual model of how QED works but what exactly does he mean by "cancel out"? Also when do you shrink and turn the little arrows and when do you "add" them together, head to tail? Do you shrink and turn arrows until you get a final arrow and then proceed to "add" several final arrows together? If so, does that mean that shrink and turning arrows gets you one step(i.e. whether a photon is reflected or transmitted) in an event(i.e. whether a photon will pass through a pane of glass)? Thanks for the help. And FYI, I'll more than likely be back with more questions as I read more :). EDIT:4 minutes after my original post and I already have another question lol. Feynman states that an electron "orbits" a proton in a hydrogen atom because the two are continuously exchanging photons. In his diagram, however, I can't find any path of a photon that would "push" the electron towards the proton. All of his pathes seem like they should "push" the electron away, not toward, the proton. I have a sneeking suspicion that this has something to do with positrons traveling backward in time but I'm not sure. Once again,thanks for the help.