Question

A concave lens has a focal length of 20 cm. At what distance from the lens a 5 cm tall object be placed so that it forms an image at 15 cm from the lens ? Also calculate the size of the image formed.

Answer 1

$\frac{1}{f}$ = $\frac{1}{v}$ - $\frac{1}{u}$
$\frac{1}{-20}$ - $\frac{1}{-15}$ = $\frac{-1}{u}$
$\frac{15-20}{300}$ = $\frac{-1}{u}$
$\frac{1}{u}$ = $\frac{-5}{300}$
u = - 60 cm

$\frac{h^{\prime }}{h}$ = $\frac{v}{u}$
$\frac{h^{\prime }}{5}$ = $\frac{-15}{-60}$
h' = $\frac{5}{4}$ cm=1.25 cm