Question

An image formed on a screen is three times the size of the object. The object and screen are 80 cm apart when the image is sharply focussed. 

 Calculate focal length of the lens.

Answer 1

Magnification (m) = 3
Object distance (u) = ?
Image distance (v) = ?
Focal length (f) = ?
Distance between image and object (v + u) = 80 cm
v = 80-u
We know that: 

`m=v/u` 

`or 3=(80-u)/u` 

`or 3u=80-u` 

`or 4u=80` 

`or u=20cm` 

thus, v=80-u 

=80-20=60 cm 

Putting these values in lens formula, we get: 

`1/v-1/u=1/f` 

`1/60-1/-20=1/f` 

`(1+3)/60=1/f`  

`f=60/4=15 cm`