Question

Define the term refractive index of a medium. Can it be less than 1? 

Define the term refractive index of a medium. Can it be less than unity?

Answer 1

The refractive index of a medium is defined as the ratio of the light's velocity in vacuum to the light's velocity in that medium.

R.I. of a medium = `"velocity of light in vacuum"/"velocity of light in that medium"`

A substance's absolute refractive index cannot be less than one since that would indicate that the speed of light is faster there than it is in empty space, which is not possible. 

In the event where the first medium is denser than the second, the relative refractive index may be less than one.