Nothing propels it at all. It actually always goes that fast, even from its creation. To explain it to you mathematically would require the use of 4-D tensor products, so I won't bother with that here. Suffice it to say, that is one of the postulates of Einstein's general relativity. Light always travels at the same speed in any reference frame.
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Refractive Index= Speed of Light in Vaccum / Speed of Light in the material
That is the speed of light.That is the speed of light.That is the speed of light.That is the speed of light.
The speed of light is not limited in a vacuum - the speed of light is fastest in a vacuum. But that is what Einstein called the "Cosmic Speed Limit" - nothing can move faster than the speed of light in a vacuum, or even quite asfast.
The speed of light is the speed at which electromagnetic waves propagate through a medium. The speed of sound is the speed at which acoustic waves propagate through a medium. As the speed of sound relies on the medium moreso than the speed of light, sound propagates much slower than light.
The index of refraction.