Question

A concave lens has 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. Draw the related ray diagram.​

Answer 1

Explanation:

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

Answer 2

f = -20cm

h1 = 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 = h2/h1

-15/-60 = h2/5

h2 = 1.25 cm

Image formed is 1.25 cm high.