When white light travels through glass, the phase velocity of each wavelength depends on the wavelength. This is the origin of dispersion and the breaking up of white light into its component colors-different wavelengths travel at different speeds and have different indices of refraction. How does vphase depend on lambda? Is dvphase/dlambda positive or negative? Therefore, is vgroup>vphase or <vphase?

When white light travels through glass, the phase velocity of each wavelength depends on the wavelength. This is the origin of dispersion and the breaking up of white light into its component colors-different wavelengths travel at different speeds and have different indices of refraction. How does vphase depend on lambda? Is dvphase/dlambda positive or negative? Therefore, is vgroup>vphase or <vphase?