A plano-convex glass lens `(mu_(g)=3//2)` of radius of curvature` R=10cm` is placed at a distance of b from a concave lens of focal length 20cm. What should be the distance a of point object O from the plano-convex lens so that position of final image is independent of b?

Explanation

Focal length of the plano-convex lens is
`(1)/(f)=((3)/(2)-1)((1)/(10)-(1)/(oo))`
or `f=20cm`
If point object Ois placed at a distance os 20 cm from the plano-convex lens, rays becme parallel and final image is formed at second focus or 20 cm from concave lens which is independent of b.