Question

A concave lens has focal length of 20 cm. At what distance from the lens can 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

Focal length of the lens=f=20cm
Object - distancee= u=?
Image distance=v=15cm
object height =$h_{o}$=5cm
Image height=$h_{i}$=?
1/f=1/v-1/u
1/u=1/v-1/f
1/u=1/15-1/20
1/u=4-3/60
1/u=1/60
u=60cm
Therefore the object must be placed at a distance of 60cm from the lens
v/u=$h_{i}/ h_{o}$
$h_{i}$=15/60*5=5/4=1.25cm