a) Consider interference of two wavefronts with equal amplitudes but slightly different wavelengths lambda = 2 pi/k: Psi(x, t) = A cos(omega_k t – kx) + A cos (omega_(k + delta k)t – (k + delta k)x) Derive and explain equations for phase and group velocity using this example b) What are phase and group velocity of light? c) Surface waves, e.g. on water, obey a more complicated dispersion, omega_k = C_k k, with c_k = B Squareroot k What are the phase and group velocity of surface ripples?