Comment by MisterMashable

I hope this clarifies the appartent contradiction. Light can't help but move at any other speed than c = 2.997 x 10^8 m/s. It can't go faster, nor slower. So how does the speed of light appear to slow down in certain circumstances? In a vacuum, the speed of light is always c. There is nothing to interfere with the propogation of the individual photons (little atoms of light) as they move through the vacuum. However, in a material such as glass the individual photons of light are absorbed and reemitted many trillions of times by the molecules making up the glass. The photons do a "stop over" and don't move at all, the velocity of these photons is zero (actually the photons temporarily don't exist except as energy absorbed by the atoms in the glass). Between lattice points or individual atoms the photons travel at velocity c. The combination of stop overs (absorbtion and reemission events) and free propogation gives the appearance light is travelling at a slower overall velocity than c.