Question

why is the value of refractive index always greater than unity?

Answer 1

refractive index of a medium is the ratio of speed of light in vacuum (or air) to the speed of light in that medium.

Refractive index of a medium= speed of light in air / speed of light in that medium

Now, as we know that speed of light is maximum in vacuum, therefore, we always get refractive index of a medium to be greater than one.

Answer 2

Refractive index is defined as the ratio of speed of light in air(vaccum) to the speed of light in that medium so speed of light is maximum in air hence value of refractive index is always greater than unity.