A plano-convex lens fits exactly into a plano-concave lens. Their plane surfaces are parallel to each other. If the lenses are made of different material of refractive indices `mu_(1)` and `mu_(2)` and R is the radius of curvature of the curved surface of the lenses, then focal length of the combination is

Options

A. `(R)/(2(mu_(1)+mu_(2)))`
B. `(R)/(2(mu_(1)-mu_(2)))`
C. `(R)/((mu_(1)-mu_(2)))`
D. `(R)/((mu_(1)+mu_(2)))`

Answer

C

Explanation

`(1)/(f)=(1)/(f_(1))+(1)/(f_(2))`
`f_(1)=(R)/((mu_(1)-1)),f_(2)=(-R)/((mu_(2)-1))`
`(1)/(f)=((mu_(1)-1))/(R)-((mu_(2)-1))/(R)=((mu_(1)-mu_(2)))/(R)`
`impliesf=(R)/((mu_(1)-mu_(2)))`