Question

An object is placed on the principal axis of convex lens of focal length $10cm$ as shown. A plane mirror is placed on the other side of lens at a distance of $20cm$. The image produced by the plane mirror is $5cm$ inside the mirror. The distance of the object from the lens is ___$cm$.

Answer 1

Answer: 30

$f=10cm$
$\frac{1}{v}-\frac{1}{u}=\frac{1}{f}$
$\frac{1}{15}-\frac{1}{-u}=\frac{1}{10}$
$\Rightarrow \frac{1}{u}=\frac{1}{10}-\frac{1}{15}$
On solving we get value of $u$ as $30cm$.