# Group and phase velocity

1. Oct 21, 2012

### Takuza

1. The problem statement, all variables and given/known data

show that the group velocity and phase velocity are related by

d vgroup = vphase - $\lambda$ ( (d vphase) / (d $\lambda$) )

2. Relevant equations
vphase = $\lambda$f
vgroup = d$\omega$/dk

3. The attempt at a solution

dw/dk = $\lambda$f - $\lambda$(d$\lambda$f/d$\lambda$)

dw/dk = $\lambda$f - $\lambda$f

Not really sure where to go with this