The weirdest 'did you know'

[Marti]

Actually light does always travel at c (300.000 km/s). What you meant is that probably it was slowed down by some kind of medium (high refraction and such)

Kind of, it got slowed down in some sort of gas

actually most mediums have a refractive index such that light appears to travel slower than in the vacuum, but its only because of the superposition of the incident electromagnetic wave and the wave emitted by the electrons oscillating because of it. So, the velocity of light calculated from the refractive index (usually the phase velocity of the wave) doesnt have much physical significance. So mediums even have refractive indexes lower than 1 which mean this velocity is greater than c.
you can also calculate the group velocity, but then again the velocity obtained is not much more than a mathematical object.
The reason why i'm explaining that is because people usually consider light as photons.
if we consider photons in the relativistic theory, and assume their velocity c is also the maximum velocity authorized in this theory (which is the case from what we know so far), then they can't travel faster or slower. they always move @ 299792458 m/s
none of these descriptions of "light" are really satisfying in every context, so it is better to assume, that it is more complicated than that.

http://www.nature.com/nature/journal/v39...7594a0.pdf