# Group and phase velocities

1. Nov 4, 2013

### Dassinia

Hello
1. The problem statement, all variables and given/known data
I have this tab
http://img18.imageshack.us/img18/3317/eyph.png [Broken]

Anf I have to find the group and phase velocities for a wavelength λ=550nm
2. Relevant equations
v(phase)= c/n

v(group)=dw/dk

v(phase)*v(group)=c²/n²

3. The attempt at a solution
I don't know how I can determine n for the wavelength of 550nm using the tab ?
I was absent the day of this course
Can I use the relation v(phase)*v(group)=c²/n² ?

Thanks !

Last edited by a moderator: May 6, 2017
2. Nov 4, 2013

### Staff: Mentor

Where do those two formulas come from? Where are they valid? Can you use them here?

Do an interpolation.
Interpolation is a much more general concept you should know.

3. Nov 4, 2013

### Dassinia

The formulas come from my book, for electromagnetic waves.
Why can't I use them ?
OK, so I'll do an interpolation !
Thanks

4. Nov 5, 2013

### Staff: Mentor

Those equations would give v(phase)=v(group) everywhere, which is not true.
Please check how they got defined, and where they are applicable.