An object is plac

If h₁ and h₂ are the sizes of the images formed in two conjugate positions, then, the size of object is given as h = √h₁h₂ = √9×1 = 3cm.

The size of the image in the two conjugate positions is

In the first case, if the image is formed, the magnification is given as . Such that, v = 3u

Also, v = 20+u, since “v” and “u” interchange the conjugate positions.

∴ 3u = 20 + u

Or 2u = 20

Or u = 10

And, v = 20+ u = 20+10 = 30 cm

Now, focal length is calculated as :

Thus, correct answer is option (d)