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:
Answer & explanation
Correct answer: option 2
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]
$\frac{1}{x}=\frac{1}{3x}-\frac{1}{-(x-5)}$
$⇒x=10cm$