Light is said to propagate as if it had a lower velocity in a material medium (like glass) than it does in vacuum.  However, we know that all matter is mostly vacuum (to within parts in 1015).  How is this possible?




It is because the photon continually re-invents itself.


When any photon exists, it travels at the speed of light.  However, as it travels through matter it occasionally encounters electrons.  A photon is an electromagnetic field particle, and it is momentarily absorbed by the electron.  The photon ceases to exist for a brief instant, and its energy is stored as a vibration of the electron.  Accelerated charges radiate, and an identical photon subsequently emerges. 


If the time for this process is short compared to the uncertainty principle, the electron remains in the same orbit, and the new photon contains all of the energy and the same oscillatory phase as the one absorbed.  Since photons are bosons, they all oscillate together in phase, so an entire ensemble of photons does this together, giving a coherent army of light that differs from the vacuum case only in that it has made many short stops along the way.