Question

At what distance should an object be placed from a convex lens of focal length 18 cm to obtain an image at 24 cm from it on the other side ? What will be the magnification produced in this case ?

Answer 1

Here, $f=18cm$

$v=24cm$

Using lens formula, $\frac{1}{v}-\frac{1}{u}=\frac{1}{f}$

$\Rightarrow \frac{1}{24}-\frac{1}{u}=\frac{1}{18}$

$\Rightarrow \frac{1}{u}=\frac{1}{24}-\frac{1}{18}=\frac{-1}{72}$

$\Rightarrow u=-72cm$

So, the object should be placed at a distance 72 cm from the lens.

Now, $m=\frac{v}{u}=\frac{24}{-72}=\frac{-1}{3}$