Question

Can the absolute value of the refractive index be less than unity?

Answer 1

The refractive index of the medium is the ratio of the speed of the light in the vacuum to the speed of the light in the air.
Refractive index is a factor that slows down the speed of light in a vacuum.
If the refractive index is less than one, it refers to the wave propagating faster than the speed of the light which is not possible.

Complete step-by-step solution:
The refractive index is the measure of the bending of the ray of light when it travels through the light. Here the light enters from one medium to another medium.
The refractive index is always greater than one.
$\mu = \dfrac{{{\text{speed in vacuum}}}}{{{\text{speed in medium}}}}$
The speed of the light only decreases when the density increases. And the vacuum is the least density medium
In a very rare case, the absolute value of the refractive index is less than one and becomes negative even in some rare cases.
For example, the refractive index of the water for X-ray is below 1.the refractive index is less than one for X-rays in any medium.
When the X-rays are entering the solids they refracted away from the normal to the refracting surface.

Note: The refractive index of a medium is described as the ratio of the speed of light in a vacuum to the speed of light in that medium.
The speed of light in a vacuum is always greater than that in any other medium. The ratio between the speed of light in a vacuum to the speed of light in any other medium is always greater than $1$.
Always, The refractive index of any medium greater than $1$.