4.4

n_{s} = \frac{n_{2}}{n_{1}} = \frac{\frac{c}{v_{2}}}{\frac{c}{v_{1}}} = \frac{v_{1}}{v_{2}}

${n_s = \frac{n_2}{n_1} = \frac{\frac{c}{v_2}}{\frac{c}{v_1}} = \frac{v_1}{v_2}}$