Question

A lens of focal length 20 cm is used to produce a ten times magnified image of a film slide on a screen. How far must the slide be placed from the lens?

Answer 1

As the image is being obtained on a screen, it is a real image and the lens is a convex lens.

$f=20cm$

$m=-10$

We know that, $m=\frac{v}{u}$

$\Rightarrow -10=\frac{v}{u}$

$\Rightarrow v=-10u$

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

$\Rightarrow \frac{1}{-10u}-\frac{1}{u}=\frac{1}{20}$

$\Rightarrow u=-22cm$

$\Rightarrow v=-10u=220cm$

So, the screen should be placed at 220 cm from the lens.