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.​

Answer 1

Answer:

Initial velocity :- -60 cm

Height :- 1.25 cm

Given:

• f = -20cm

• h1 = 5 cm

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

Solution:

→ 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.