Question

Is it possible for a material to have a refractive index less than `1.000`?

Answer 1

In a substance with a refractive index less than 1.000, light would travel faster than the speed of light. As far as we know, light cannot, so a refractive index lower than 1.000 cannot occur.

Explanation:

Empty space has a refractive index of 1.000000... Air has a refractive index of about 1.000293.

Light travels through empty space at the speed of light, `3.0xx10^8` `ms^-1`. If something had a refractive index less than 1.000, Snell's Law states that light would travel at a higher velocity than that.

As far as we know, light cannot travel faster than the speed of light.

There are good theoretical reasons for this, relating to the electromagnetic nature of light, and to the permeability and permitivity of free space.

If light cannot travel faster than `3.0xx10^8` `ms^-1`, no material can have a refractive index less than 1.000.