The optimum blade velocity is given by

Question

If V_N and a are the nozzle exit velocity and nozzle angle in an impulse turbine, the optimum blade velocity is given by                                                                                               

      (a) V_N cos 2$\alpha$                                                     (b) V_N sin 2$\alpha$

      (c) $\frac{V_{N}\cos \alpha }{2}$                                                        (d) $\frac{V_{N}\sin \alpha }{2}$