Phase and Group Velocity Problem

[coltin.walsh]

Homework Statement

see:

http://www.flickr.com/photos/55153239@N03/6371848305/ [Broken]

Homework Equations

vp = w/k

vg = dw/dk

The Attempt at a Solution

I know the relationship of:

vg = vp(1+lambda/n * (dn/dlambda))

but i don't know where to go from here..

 

[rude man]

1. given n(λ), what is phase velocity vp of light of wavelength λ?
2. as you state, w/k = vp. So how about rewriting w in terms of λ and n(λ), using (1) above?
3. use chain rule dw/dk = dw/dλ*dλ/dk