• # question_answer A concave lens forms the image of an object such that the distance between the object and the image is 10 cm and the magnification produced is 1/4. The focal length of the lens is: A)  8.6 cm                       B)  6.2 cmC)  10 cm                        D)  4.4 cm

Concave lens forms the virtual image of a real object. So let $u=-4x$and $v=-x$then $3x=10\,cm$or $x=\frac{10}{3}\,cm$ $\therefore$$u=\frac{-40}{3}cm$and $v=-\frac{10}{3}\,cm$ Substituting in $\frac{1}{f}=\frac{1}{v}-\frac{1}{u},$we get $\frac{1}{f}=-\frac{3}{10}+\frac{3}{40}$or $f=-\frac{40}{9}=-4.4\,cm.$ Hence, the correction option is [d].