Question

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

Answer 1

$f =10\, cm$
$ \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 $30 \,cm$.