One plano-convex and one plano-concave lens of same radius of curvature ‘R’ but of different materials are joined side by side as shown in the figure. If the refractive index of the material of 1 is μ1 and that of 2 is μ2, then the focal length of the combination is:

• A. \(\frac{{\rm{R}}}{{{\mu _1} - {\mu _2}}}\)
• B. \(\frac{{2{\rm{R}}}}{{{\mu _1} - {\mu _2}}}\)
• C. \(\frac{{\rm{R}}}{{2\left( {{\mu _1} - {\mu _2}} \right)}}\)
• D. \(\frac{{\rm{R}}}{{2 - \left( {{\mu _1} - {\mu _2}} \right)}}\)