# Homework Help: Derive the Sellmeier equation

1. Aug 28, 2011

### lillemy

1. The problem statement, all variables and given/known data
Derive the Sellmeier equation
$n^{2} = 1 + \frac{A\lambda^{2}_{vac}}{\lambda^{2}_{vac}-\lambda^{2}_{0,vac}}$
from
$(n+i\kappa)^{2}= 1 + \frac{\omega^{2}_{p}}{\omega^{2}_{0}- i\omega\gamma - \omega^{2}}$

for a gas or glass with negligible absorption (i.e. $\gamma$ ≈ 0, valid far
from resonance $\omega_{0}$, where $\lambda_{0,vac}$
corresponds to frequency $\omega_{0}$ and A is a constant.

2. Relevant equations
$\omega = \frac{2\pi c}{\lambda_{vac}}$

$\omega^{2}_{p}= \frac{Nq^{2}_{e}}{\epsilon_{0}m_{e}}$

3. The attempt at a solution

Since the absorption is negligible, $\gamma = 0$ we can drop the imaginary part , and I will substitute directly for $\omega$ and $\omega_{p}$ from the above equations. It gives this result:

$1+ \frac{\lambda^{2}_{vac}\lambda^{2}_{0,vac}\frac{Nq^{2}_{e}}{4\pi^{2}c^{2}\epsilon_{0}m_{e}}}{\lambda^{2}_{vac}-\lambda^{2}_{0,vac}}$

i.e. everything is ok expect that i have on extra of $\lambda^{2}_{0,vac}$ in the numerator. What have I done wrong? Very thankful for all help:)

2. Dec 6, 2014

### Brian Coffey

I know the thread is 3 years old but any idea on this question? Have a similiar problem, appreciate any help

3. Dec 6, 2014

### ehild

Nothing is wrong. That "extra" $\lambda^{2}_{0,vac}$ is included into the constant A.

4. Dec 6, 2014

### Brian Coffey

That's what I was thinking but wasn't sure since that term appeared elsewhere in the formula, thanks for your help!