How Do You Calculate Group and Phase Velocities for a 550nm Wavelength?

[Dassinia]

Hello

Homework Statement

I have this tab
http://img18.imageshack.us/img18/3317/eyph.png

Anf I have to find the group and phase velocities for a wavelength λ=550nm

Homework Equations

v(phase)= c/n

v(group)=dw/dk

v(phase)*v(group)=c²/n²

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 !

 

[mfb]

v(phase)= c/n
v(phase)*v(group)=c²/n²

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

I don't know how I can determine n for the wavelength of 550nm using the tab ?

Do an interpolation.

I was absent the day of this course

Interpolation is a much more general concept you should know.

 

[Dassinia]

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

 

[mfb]

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