[Steely Tom]

Light comes in packets (it's not quite right to call them "particles") called "photons." Each photon consists of a little electric field and a little magnetic field that kind of whirl around each other, maintaining perfect dynamic equilibrium; as one decays, the other builds, and vice-versa, until the photon encounters something that absorbs or scatters it.

The equations that describe this marvelous dance of fields were discovered by James Clerk Maxwell, who first published his insight in 1861, in the early months of our Civil War.

Maxwell's equations only generate the intertwined solution combining magnetic and electric fields when the reference frame is moving at the speed of light. For any other velocity, the equilibrium breaks down, the bubble pops, the photon dissipates its little quantum of energy.

Photons can move through materials other than the vacuum. When this happens, the speed of light in those materials differs from that of the speed of light in a vacuum.

In any medium other than vacuum, the speed of light is slower. No one has ever found a material in which the speed of light exceeds measured in a vacuum.

[jpsb]

Nice explanation.