Question:
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?
Answer:
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.