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I know that the group velocity of a light pulse is defined as

$$\begin{split}v_g&=v_p\left(1+\frac{\lambda}{n}\frac{dn}{d\lambda}\right)\\ &=\frac{c}{n}\left(1+\frac{\lambda}{n}\frac{dn}{d\lambda}\right).\end{split} \tag{1}$$

On the other hand, it is also defined as

$$\begin{split}v_g &= \frac{c}{\left(n-\lambda\frac{dn}{d\lambda}\right)}\\ &=\frac{c}{n}\left(1-\frac{\lambda}{n}\frac{dn}{d\lambda}\right)^{-1}.\end{split}\tag{2}$$

That leads to the equation

$$0=\frac{\lambda^2}{n^2}\left(\frac{dn}{d\lambda}\right)^2.\tag{3}$$

After neither $\lambda$ nor $n$ is $0$, does that mean that $\left(\frac{dn}{d\lambda}\right)^2=0$? And how can I justify that?
Source: http://en.wikipedia.org/wiki/Group_velocity#Other_expressions
Alternative explanation: Wiki is crap?

Qmechanic
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arc_lupus
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3 Answers3

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The first equation correctly states that $$ v_g = \frac{c}{n}\left(1 + \frac{\lambda}{n}\frac{dn}{d\lambda}\right). $$ But if you look at the wikipedia page that you linked to, you'll see that the second equation should read $$ v_g = \frac{c}{n}\left(1 - \frac{\lambda_0}{n}\frac{dn}{d\lambda_0}\right)^{-1}, $$ where $\lambda_0$ is the wavelength in vacuum, and $$ n = \frac{\lambda_0}{\lambda}, $$ where $\lambda$ is the wavelength in the medium. The second equation can be derived from the first as follows: $$ \begin{align} v_g &= \frac{c}{n}\left(1 + \frac{\lambda}{n}\frac{dn}{d\lambda}\right)\\ &= \frac{c}{n}\left[1 + \frac{\lambda}{n}\left(\frac{d\lambda}{dn}\right)^{-1}\right]\\ &= \frac{c}{n}\left[1 + \frac{\lambda_0}{n^2}\left(\frac{d(\lambda_0/n)}{dn}\right)^{-1}\right]\\ &= \frac{c}{n}\left[1 + \frac{\lambda_0}{n^2}\left(-\frac{\lambda_0}{n^2} + \frac{1}{n}\frac{d\lambda_0}{dn}\right)^{-1}\right]\\ &= \frac{c}{n}\left[1 + \left(-1 + \frac{n}{\lambda_0}\frac{d\lambda_0}{dn}\right)^{-1}\right]\\ &= \frac{c}{n}\left[\frac{n}{\lambda_0}\frac{d\lambda_0}{dn}\left(-1 + \frac{n}{\lambda_0}\frac{d\lambda_0}{dn}\right)^{-1}\right]\\ &= \frac{c}{n}\left[\left(-\frac{\lambda_0}{n}\left(\frac{d\lambda_0}{dn}\right)^{-1} + 1\right)^{-1}\right]\\ &= \frac{c}{n}\left(1 - \frac{\lambda_0}{n}\frac{dn}{d\lambda_0}\right)^{-1}. \end{align} $$

Pulsar
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Group velocity, for any kind of wave, is defined as $$\boxed{v_g=\frac{\mathrm dω}{\mathrm dk}}.$$

Phase velocity is defined as $v_p=\dfrac ωk$, and refraction index as $n=\dfrac c{v_p}$. So $ω=c\,\dfrac kn$. Hence, $$\mathrm dω=c\left(\frac{\mathrm dk}n-\frac{k\,\mathrm dn}{n^2}\right)=\frac{c\,\mathrm dk}{n}\left(1-\frac kn\frac{\mathrm dn}{\mathrm dk}\right).$$

Given that $k=\dfrac{2π}λ$, $\dfrac{\mathrm dk}k=-\dfrac{\mathrm dλ}λ$ (logarithmic differentiation), so that $$v_g=\frac{\mathrm dω}{\mathrm dk}=\frac cn\left(1+\frac λn\frac{\mathrm dn}{\mathrm dλ}\right).$$

Edit: this demonstration shows the first equation you cite is correct, the second is not.

L. Levrel
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  • My point is that I do not have k in the first equation, but $\lambda$ instead – arc_lupus Apr 13 '16 at 08:47
  • Please read the whole answer. Group velocity is defined as I stated, period. k=2π/λ so you can reformulate this with λ if you like, vg=-λ²/2π*dω/dλ. – L. Levrel Apr 13 '16 at 08:51
  • That means that Wikipedia is apparently incorrect... – arc_lupus Apr 13 '16 at 09:12
  • Edit 2: Found a solution for both possibilities, your answer might need an edit. – arc_lupus Apr 13 '16 at 09:38
  • The group velocity is a vector, defined as:$$\mathbf{V}{g} = \partial{\mathbf{k}} \ \omega\left(\mathbf{k}\right) = \frac{c}{\omega} \left[ \hat{\mathbf{k}} \frac{\partial \omega}{\partial n} + \hat{\boldsymbol{\theta}} \frac{1}{n} \frac{\partial \omega}{\partial \theta} \right]$$ – honeste_vivere Apr 13 '16 at 11:24
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Ok, I found an answer on my own:
For the first definition the answer from L. Levrel is sufficient. For the second definition one has to follow the following set of equations: $$\begin{split} \frac{1}{v_g}&=\frac{dk}{d\omega}\\ &=c^{-1}\left(n+\omega\frac{dn}{d\omega}\right)\\ &=c^{-1}\left(n-\lambda\cdot\frac{dn}{d\lambda}\right)\\ \Rightarrow v_g&=\frac{c}{n\left(1-\frac{\lambda}{n}\frac{dn}{d\lambda}\right)} \end{split}$$ which is what I wanted to get.
How to get from $\omega$ to $\lambda$: $$\begin{split} \omega&=\frac{2\pi c}{\lambda}\\ \Rightarrow\frac{dn}{d\omega}&=\frac{dn}{d\lambda}\frac{d\lambda}{d\omega}\\ &=\frac{dn}{d\lambda}\frac{d\lambda}{d\frac{2\pi c}{\lambda}}\\ &=\frac{1}{2\pi c}\frac{dn}{d\lambda}\frac{d\lambda}{d\frac{1}{\lambda}}\\ &=-\frac{\lambda^2}{2\pi c}\frac{dn}{d\lambda}\\ &=-\frac{\lambda}{\omega}\frac{dn}{d\lambda}\end{split}$$

arc_lupus
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    I'm curious to read how you get line 3 from line 2, that is, how you change variable $ω$ into $λ$. Remember that $ω=2πc/nλ$. Your answer might need an edit (or deletion) :-) – L. Levrel Apr 13 '16 at 19:37
  • @L.Levrel: I added an explanation, I hope that helps. In general I had the impression that $\omega$ is not influenced by the refractive index, but the phase velocity is (by $v_p = \frac{\omega}{k_0\cdot n}$) – arc_lupus Apr 13 '16 at 20:38
  • Suggestion to the answer (v2): Replace all appearances of $\lambda$ with the vacuum wavelength $\lambda_0$. – Qmechanic Apr 13 '16 at 22:18
  • @L.Levrel: Do you have a source for $\omega=\frac{2\pi c}{n\lambda}$? I could not find anything for that... – arc_lupus Apr 14 '16 at 06:44
  • By definition, $v_p=ω/k$. Since $v_p=c/n$ by definition of $n$, you find $ω=kc/n$. Of course $λ$ depends on $n$. See also Pulsar's answer. – L. Levrel Apr 14 '16 at 19:40