The plane surface of a plano-convex lens of focal length 60 cm is silvered. A point object is placed at a distance 20 cm from the lens. Find the position and nature of the final image formed.

Explanation

Let f be the focal length of the equivalent spherical mirror,
we have
or,`1/f=1/f_(1) + 1/f_(m) + 1/f_91)`
Here `f_(1)=+60cm`
`f_(m)=infty therefore 1/f=2/60 + 1/infty =1/30` or f=+30cm
The problem is reduced to a simple case where a point object is placed in front of a convex.
Now, using the mirror formula
`1/u + 1/v =1/f`, we have `implies 1/(-20) + 1/v=1/(-30) impliesv=60cm`
The image is erect and virtual.