A converging lens forms a three times magnified image of an object, which can be taken on a screen.

Question

A converging lens forms a three times magnified image of an object, which can be taken on a screen. If the focal length of the lens is 30 cm , then the distance of the object from the lens is\

Professor PC Thomas Classes · Answer

(d):Given, converging lens image is three times magnified, focal length, $f=30 \mathrm{~cm}$.\ As we know, when a convex lens forms a magnified and real image, then the image is inverted i.e., magnification must be negative.\ Therefore, magnification, $m=-3$\ Using lens formula, $\frac{1}{v}-\frac{1}{u}=\frac{1}{f}$\ we get, $\frac{v}{f}=1-m \Rightarrow \frac{v}{f}=1-(-3) \Rightarrow \frac{v}{30}=4 \Rightarrow v=120 \mathrm{~cm}$\ Using, $m=\frac{v}{u} \Rightarrow-3=\frac{120}{u} \Rightarrow u=-40 \mathrm{~cm}$\ Thus, object distance is at -40 cm from the lens.\