An object is placed at a distance of 5 cm beyond focus on the principal axis of a convex lens. The image is formed at a distance of 3 f on the other side of the lens. The value of focal length (f) is:

10 cm

The correct answer is Option (2) → 10 cm

Let magnitude of focal length of convex lens = x

then,

Object distance, $u=-(x-5)$

Image distance, $v=3x$

as,

$\frac{1}{f}=\frac{1}{v}-\frac{1}{u}$ [Lens formula]

$\frac{1}{x}=\frac{1}{3x}-\frac{1}{-(x-5)}$

$⇒x=10cm$