You might want to look into the theory behind fiber optics a bit ... I left out the deviation due to path length (bouncing) as that obviously has a lot of varia

You might want to look into the theory behind fiber optics a bit ... I left out the deviation due to path length (bouncing) as that obviously has a lot of variations (including wavelength). I've also ignored SBS and other unrelated sources of scatter. It's pretty common to hear people refer to the "speed-of-light in a vacuum". If the speed of light were a constant, why would you need to specify the vacuum? It's also worth noting that electrons don't move through conductors at the speed-of-light either - in the cable television industry, the best hard-line coaxial cable was about 0.85.