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 !