The refractive indices of medium $(1)$ and medium $(2)$ are $\eta_{1}$ and $\eta_{2}$ respectively,and the velocities of light in these media are $v_{1}$ and $v_{2}$ respectively. What is the formula to find the refractive index of medium $(2)$ with respect to medium $(1)$,denoted as $\eta_{21}$?

• A
  $\frac{v_{1}}{v_{2}}$
• B
  $\frac{\eta_{1}}{\eta_{2}}$
• C
  $\frac{v_{2}}{v_{1}}$
• D
  $\eta_{1} \eta_{2}$