**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:

best answer for A concave lens has a focal length of 20 cm.

f = -20cm

h_{1 }= 5 cm

v = -15 cm (Concave lens forms virtual image)

1/v – 1/u = 1/f

1/-15 – 1/u = 1/-20

1/u = -(1/15) + 1/20

1/u = -1/60

u = -60 cm

object should be placed 60 cm to the left of the lens.

m = v/u = h_{2}/h_{1}

-15/-60 = h_{2}/5

h_{2} = 1.25 cm

Image formed is 1.25 cm high.