An object placed at 40 cm from a concave lens produces an im

An object placed at 40 cm from a concave lens produces an image at 13.3 cm from the lens. What is the focal length of the lens?

Solution

Object distance u = -40 cm

Image distance v = - 13.3 cm

Since concave lens always forms virtual image.

from the relatioon ( 1/u ) +(1/v ) = (1/f)

                         ( 1/-40)+(1/-13.3) = (1/f)

               ( -13.3 -40 ) /(40 x 13.3) = (1/f)

                                                f = (40x13.3) /( -13.3 -40)

                                                  = -9.981 cm