The ultimate answer is that, in our Universe, it just is. There's no reason it HAS to be that way, but it just is -- whether we like it or not.
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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.
Refractive Index= Speed of Light in Vaccum / Speed of Light in the material
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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.