question_answer

A plano-convex lens (focal lengthy\[{{f}_{2}}\], refractive index \[{{\mu }_{2}},\]radius of curvature R) fits exactly into a plano-concave lens (focal length \[{{f}_{1}},\]refractive index \[{{\mu }_{1}},\]radius of curvature -R). Their plane surfaces are parallel to each other. Then, the focal length of the combination will be-               [JEE Main 12-Jan-2019 Evening]

A) \[{{f}_{1}}+{{f}_{2}}\]

B) \[\frac{R}{{{\mu }_{2}}-{{\mu }_{1}}}\]

C) \[{{f}_{1}}-{{f}_{2}}\]

D) \[\frac{2{{f}_{1}}{{f}_{2}}}{{{f}_{1}}+{{f}_{2}}}\]

Correct Answer: B

Solution :

[b] The focal length of the two lens is given

So, the focal length of the lens combination is