Question

The subject is at a distance of d = 1.8 meters from the lens. Determine the focal length of the lens if the image size is 5 times the size of the subject.

Answer 1

Object distance, $u=-1.8\space m$

Image distance $=v$

Magnification, $m=5$

$m=\dfrac{v}{u}$

$v=-5\times 1.8=-9\space m$

Lens formula,

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

$\dfrac{1}{f}=-\dfrac{1}{9}+\dfrac{1}{1.8}$

$f=2.25\space m$