Yesterday in my physics lesson we were covering electromagnetism and my teacher mentioned the speed of light in a material was equal to the reciprocal of the square root of the magnetic permeability and electric permittivity of the material. I'd come across this before but I had always assumed this was because any photon would be absorbed/emitted in a random fashion, increasing the time it took to propogate the material. However my teacher said that this value was the actual, true speed of light and that the speed of light was only c in a perfect vacuum. I find this incredibly hard to believe as photons are massless and if you start out with Einstein's famous equation (the full one) you end up with f*lambda = c. Treating light as a wave then f*lambda = velocity of the wave and therefore the photon should travel at c, through a vacuum or through molasses or whatever. So am I just deluded or was my teacher wrong? Cheers, Just some guy.