Question

A converging lens with a focal length of 40 cm is used to form an image on a screen that is 2.8 m behind the lens. At what distance in front of the lens must the object be placed?

Answer 1

here,

the focal length , f = 40 cm

the image distance , di = 2.8 m = 280 cm

let the object distance be do

using the lens formula

1/f = 1/di + 1/do

1/40 = 1/280 + 1/do

do = 46.7 cm

the object is placed 46.7 cm in front of the lens