Question

A convex lens produces an inverted image magnified three times of an object placed at a distance of 15 col from it. Calculate focal length of the lens.

Answer 1

Here, $u=-15cm$

$m=\frac{v}{u}=-3$ (Since the image is inverted)

$\Rightarrow v=-3u=45cm$

Now using the lens formula, $\frac{1}{v}-\frac{1}{u}=\frac{1}{f}$

$\Rightarrow \frac{1}{45}-\frac{1}{-15}=\frac{1}{f}$

$\Rightarrow f=\frac{45}{4}=11.25cm$