so what i am about to write is quite possibly wrong, but this is just how i was thinking about why the speed of light is different in a vacuum as it is in glass or water etc. i am writing this to see whether my train of thought is on the right lines, and if it isn't to hopefully get an insight into the way i should think about it. if the speed of light, c, is 3x10^8 ms-1 approx. in a vacuum (free space), because there is nothing there for the light to 'knock into' or interact, and therefore nothing to slow it down, i think of it as a kind of 'resistance' (i know this isn't a correct term to use here, but they're similar ideas). and in air the speed of light is somewhat slower due to the fact that there are particles moving about in this free space, i might assume, that it is these particles that slow the light down. however what i was thinking is that air is mostly free space anyway, so does a photon actually travel at speed c, for most of the time, but it is 'held up' by some kind of interaction with an electron? so it's actual speed doesn't really change, but it's trip time increases due to these interactions... so when a photon travels from A-D, it must 'stop' at B and C along the way, but it does travel from A-B at speed c and the same for B-D. it is just the fact that whatever happens at those points takes some time to occur please let me know if that's a sensible idea or not. thanks, lntz.