Question

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

Answer 1

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

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 = 10.8 cm