What is index of refraction and what does it have to do with the speed of light?

Index of refraction is the ratio of the velocity of light in medium to the velocity of light in vacuum.

OR:

Index of refraction is the ratio of the sine of the angle of incidence to sine of the angle of refraction of light propagating from one medium to another.

When the refractive index is greater, the speed of light is greater as it moves from the medium to vacuum.

the speed of light =

299 792 458 m / s

Light from a stationary source travels at 300,000 km/sec (186,000 miles/sec).

The speed of light is measured to have the same value of c = 3x108 m/s no matter who measures it. The speed of light in a vacuum is 186,282 miles per second (299,792 kilometers per second), and in theory nothing can travel faster than light. In miles per hour, light speed is, well, a lot: about 670,616,629 mph. If you could travel at the speed of light, you could go around the Earth 7.5 times in one second. The speed of light in a vacuum is usually denoted by a lowercase c, for "constant" or the Latin celeritas (meaning "swiftness, celerity"). In 1856, Wilhelm Eduard Weber and Rudolf Kohlrausch had used c for a different constant that was later shown to equal √2 times the speed of light in a vacuum. Is there such a thing as the speed of dark? In a 2013 study, scientists determined that dark matter should have a speed of 54 meters per second, or 177 feet -- slow compared to the speed of light. Nothing can move faster than the speed of light. When Einstein set forth his theory of relativity, this was his inviolable postulate: that there was an ultimate cosmic speed limit, and that only massless particles could ever attain it. All massive particles could only approach it, but would never reach it.

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