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

Step 1, Given data

Focal length, f = 20 cm

Object distance = -u

Image distance = v

Magnification, m = -10 (negative sign implies real image)

Step 2, Finding distance of slide

We know,

$Magnificationm=\frac{v}{u}$

Or,

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

$Or,v=10u$

We know from the lens formula

$\frac{1}{v}-\frac{1}{u}=\frac{1}{f}$

Putting all the values

$\frac{1}{10u}+\frac{1}{u}=\frac{1}{20}$

After solving

u = 22 cm

Hence, the slide is placed at a distance of 22 cm from the pole of the lens.