Question

what is refractive index

Answer 1

the ratio of velocity of light in vacuum to the velocity of light in medium is called as refractive index. It is denoted by letter n. it has no units

n = c/v
it is a dimensionless quantity because it is a ratio of same physical quantities.

Answer 2

Refractive index is defined as the ratio of the speed of light in first medium to the speed of light in second medium in the given substance.