3. the phase speed v, of light in a certain wavelength range in a medium is given by the following expression , a here a is the wavelength, c is speed of light,

3. The phase speed V, of light in a certain wavelength range in a medium is given by the following expression , A Here A is the wavelength, C is speed of light, and A and B are constants. (a) Find an expression of the group speed in terms of wave vector k. (b) Taking A - 1.7 and B- um), calculate the group and phase speed of light for a wavelength of 400 nm. Which of the two speeds is physically important and why? 0.01 47