A plano convex lens of material of refractive index $\mu_1$ exactly fits into a plano concave lens of material of refractive index $\mu_2$. If $R$ is the radius of curvature of the curved surfaces of the lenses and the plane surfaces of the lenses are parallel, the focal length of the combination is

• A

  $\begin{aligned} & \mathrm{R}\left(\mu_1-\mu_2\right) \end{aligned}$
• Correct answer
  B

  $\begin{aligned} & \frac{\mathrm{R}}{\mu_1-\mu_2}\end{aligned}$
• C

  $\begin{aligned} & \frac{\mu_1-\mu_2}{R}\end{aligned}$
• D

  $\begin{aligned} & \frac{1}{R\left(\mu_1-\mu_2\right)}\end{aligned}$

Hence, the correct answer is option: 'B'.