"refractive index" or index of refraction n of a substance (optical medium) is a dimensionless number that describes how light, or any other radiation, propagates through that medium.
Applicable units are those of quantitykind:RefractiveIndex
http://en.wikipedia.org/wiki/Refractive_index
"refractive index" or index of refraction n of a substance (optical medium) is a dimensionless number that describes how light, or any other radiation, propagates through that medium.
$n = \frac{c_0}{c}$, where $c_0$ is the speed of light in vacuum, and $c$ is the speed of light in the medium.