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What is the refractive index?

Answer 1

It is a pure number that gives us the value of the ratio between the speed of light in vacuum and the speed of light inside a medium.

In passing from vacuum to a medium light slows down. So if we cal the speed of light in vacuum #c# and in the medim #v#, then the refractive index #n# will be:
#n=c/v#
It is a way to carachterize a phisical medium (say, water) with respect to the change in speed of light it produces so that you'll have different values for different materials:

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Answer 2

Josiah Adams

The refractive index, usually denoted by the symbol "n," is a measure of how much light bends or refracts when it passes from one medium into another. It is defined as the ratio of the speed of light in a vacuum to the speed of light in the medium.

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Answer from HIX Tutor

When evaluating a one-sided limit, you need to be careful when a quantity is approaching zero since its sign is different depending on which way it is approaching zero from. Let us look at some examples.