Question

what do you mean by refractive index?

Answer 1

refractive index is the capacity of medium to bend the light.

refractive index of medium 1 with respect to medium 2 is given by
velocity of light in medium 1
divided by
velocity of light in medium 2

Answer 2

a measure of extent to which a substance slows down light waves passing through it the index of refractive of a substance is equal to the ratio of the velocity of light in a vaccum to its speed in that substance its value determine the extent to which light is refracted when entering or leaving the substance.