Question

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

Answer 1

Correct Answer - a.
`(1)/(F)=(mu-1)((1)/(oo)+(1)/(R))+(mu_(2)-1)((1)/(-R)-(1)/(oo))`
`=(mu_(1)-mu_(2))/(R)` or `F=(R)/(mu_(1)-mu_(2))`