Dispersion relation — UPSC CSE 1986 Physics PYQ

The angular frequency, \omega, of a wave is related to the wave vector, k, by the relation \omega = \omega_{0} \sin\frac{ka}{2} \text{ for } 0 < k > \frac{\pi}{a} Determine the phase and group velocities of this wave and draw a rough sketch to show their variation with k(0 < k < \pi/a)