Question

A concave lens produces an image 20 cm from the lens of an object placed 30 cm from the lens. The focal length of the lens is:
(a) 50 cm
(b) 40 cm
(c) 60 cm
(d) 30 cm

Answer 1

60 cm
Object distance from the lens, u =-30 cm
Image distance from the lens, v =-20 cm 

Using the lens formula, we get: 

`1/f=1/v-1/u` 

`1/f=1/-20-1/-30` 

`1/f=1/-20+1/30` 

`1/f=(-3+2)/60=1/60` 

∴ f =- 60 cm

Therefore, the focal length of the lens is 60 cm.