Question

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

Answer 1

Answer:

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