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

Given:
m = -10 (negative sign implies real image)
f = 20 cm
Object distance = -u
Lens formula is given by:  

`1/f=1/v-1/u` 

Magnification in case of convex lens is given by: 

`m=v/-u` 

`v=mu` 

`v=(-10)xx(-u)=10u`

Putting these values in lens formula, we get:  

`1/f=1/v-1/u` 

`1/20=1/(10u)+1/u` 

`1/20=1+10/(10u)` 

`1/20=11/(10u)` 

`10u=220` 

`u=220/10=22` cm 

Thus, object distance = -u = -22 cm
So, the slide is placed at a distance 22 cm from the poles of the lens.