Question

A lens of focal length $f$ forms an image of magnification $n$, The distance of object from the lens in terms of $f$ and $n$ is

• A. $(1+n)f$
• B. $\frac{nf}{(1-n)}$
• C. $\frac{(1-n)f}{n}$
• D. $\frac{f}{n}$

Answer 1

The correct option is C $\frac{(1-n)f}{n}$

We have,
magnification $n=\frac{f}{f+u}$
Here $u$ is distance of object from lens and
$f$ is focal length

$\Rightarrow f+u=\frac{f}{n}$
$\Rightarrow u=\frac{f}{n}-f$
$\Rightarrow u=f\left(\frac{1-n}{n}\right)$