# Conditions for having refractive index less than unity?

Is it possible to have an index of refraction, $n < 1$, giving the "illusion" of a speed greater than $c$.

As far as I understand it, is only the phase-velocity (which does not carry any information) that exceeds the speed of light.

But what I would really like to know is, under what conditions it is possible to have $n<1$?

• You'll need some unobtanium crystals. Other than n= -1 (mirror reflection) and n<-1 for certain specially created optical materials, n>=1 is required by definition, i.e. n=1 in a vacuum. – Carl Witthoft Jun 6 '15 at 13:56
• If I recall correctly, there are meta-materials that can mimic n<1, but I am not terribly familiar with the details & don't have much to give beyond that. – Kyle Kanos Jun 6 '15 at 14:17
• en.m.wikipedia.org/wiki/Negative_refraction...I wonder is this what you mean. – user81619 Jun 6 '15 at 14:18
• I am talking about 0<n<1. – Nillo Jun 6 '15 at 14:19
• This can occur close to resonance frequencies, for absorbing media, in plasmas, and for X-rays wiki – user46925 Jun 6 '15 at 14:22

I have been reaserching some more, and what I have found is something about waves in dispersive media. (The wave speed depends on the frequency of the wave). Source: (http://www.acs.psu.edu/drussell/Demos/Dispersion/dispersion.html) Apparently when the wave is traveling with a frequency close to the resonans frequency $n$ gets smaller than $1$. But only the group velocity exceeds $c$.