A "fact" is basically anything you can state, especially if such a statement is true. Since you can say "light has a speed", and since that is true, it follows that it's a fact.
Light through a "vacuum" is the meaning of "the speed of light." Light through glass is slower.
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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.