A lens is formed by placing the plane faces of two identical plano-convex lenses (each of focal length 40 cm) touching each other. It is used to obtain a real, inverted image of the same size as the object. The distance of the object from the lens is:

40 cm

The correct answer is Option (2) → 40 cm

When two lenses are kept in contact, the effective focal length $(F_{eff})$ is -

$(F_{eff})=\frac{1}{F_1}+\frac{1}{F_2}$

$(F_{eff})=\frac{1}{40}+\frac{1}{40}=\frac{2}{40}=\frac{1}{20}$

$(F_{eff})=20cm$ [Both lens had $f = 40 cm$ and identically]

Now,

Since, $v=-u$ a image should be inverted and be of the same size.

$(F_{eff})=\frac{1}{u}-\frac{-1}{u}$

$\frac{1}{20}=\frac{2}{u}$

$=u=40cm$