# Why does refractivity not depend on density; Lorentz-Lorenz

1. Apr 17, 2015

### Soren

I must be stupid but my textbook states that the refractive index of a media does not depend on the density according to the Lorentz-Lorenz formula (LL)
$$(\frac{n^2-1}{n^2+2}) = \frac{4\pi Ne^2}{ 3m} \frac{1}{\omega_0^2 \omega^2}$$
Specifically it (Physical Optics, Akhmanov Nikitin pp.367) says: ".. since ρ ~ N, it follows from (LL formula) that the quantity
$$r =(\frac{n^2-1}{n^2+2}) \frac{1}{\rho}$$
should not depend on density"
Are they simply telling me that ρ appears on both sides of the equation?

2. Apr 17, 2015

### nasu

They don't say that the index of refraction does not depend on density. At least not in the quote you give.
That "r" is not the index of refraction.

3. Apr 17, 2015

### Soren

no you are right, r is called the specific refractivity of the substance. I suppose that is what they are trying to tell me, that specific refractivity (r) does not depend on density, whereas the refractive index of a substance does.

Last edited: Apr 17, 2015