The solution of the problem 3.52 cr 3 from the problem

The solution of the problem 3.52 cr 3 from the problem

Task 3.52
Calculate the difference between the phase and group velocity for light with a wavelength of 0.768 μm in the glass,
If it is known that the refractive index for this wavelength is 1.511, and for a wavelength of 0.656
M, it is 1.514.