# Dispersion and refractive indices

1. Jan 15, 2014

### user3

If different frequencies of light have different refractive indices for the same material and travel at different speed in the same material, isn't it inaccurate to say that the speed of light through a certain material is c/n, where n is the "standard" refractive index?

2. Jan 15, 2014

### clem

Yes, but it is usually a reasonable approximation.
The dependence of n on frequency is why the 'group velocity' differs from the 'wave velocity'.
A short wave packet would tend to spread because of this depedence of n on frequency.

3. Jan 15, 2014

### DrDu

Well, if there is dispersion, you would rather say that $c(\omega)=c_0/n(\omega)$. Often, people take it for granted that c and n are functions of $\omega$ and don't mention it explicitly.

4. Jan 15, 2014

### sophiecentaur

"n" is specified at a certain wavelength. Visible light covers an octave of frequencies so it is hardly surprising that it interacts with transparent substances differently over that range.