Question

The figure given below shows a point object and a converging lens. The final image is formed at

Answer 1

Applying lens formula,
$\frac{1}{v}-\frac{1}{u}=\frac{1}{f}$
Here, $u=-15\text{cm},f=+10\text{cm}$
$\Rightarrow \frac{1}{v}-\frac{1}{(-15)}=\frac{1}{10}$
or, $\frac{1}{v}=\frac{1}{10}-\frac{1}{15}=\frac{3-2}{30}$
$\therefore v=+30\text{cm}$

Why this question? Bottom line: Always remember that we take sign convention for second focus or principal focus.