Question

A point object is placed at a distance of $18cm$ from a convex lens on its principal axis. Its image is formed on the other side of the lens at $27cm$. Calculate the focal length of the lens.

• A. 10.2 cm
• B. 10.5 cm
• C. 10.8 cm
• D. 11.5 cm

Answer 1

The correct option is C

10.8 cm

According to the sign convention, the object is placed on the left side of the lens, therefore object-distance is negative $\Rightarrow u=-18cm$.
Since the image is formed on the other side, the image-distance is positive $\Rightarrow v=+27cm$.

Using len's formula, $\frac{1}{v}-\frac{1}{u}$ = $\frac{1}{f}$, we have

$\frac{1}{+27}-\frac{1}{-18}$ = $\frac{1}{f}$

$f=10.8cm$

Hence, the focal length is 10.8 cm.