To get an image of magnification -1 on a screen using a lens of focal length 20 cm , the object dist

Question

To get an image of magnification -1 on a screen using a lens of focal length 20 cm , the object distance must be\

Professor PC Thomas Classes · Answer

(c):Magnification, $m=\frac{v}{u}=-1 \Rightarrow v=-u$ Also, $f=20 \mathrm{~cm}$\ Using lens formula, we have\ $\frac{1}{v}-\frac{1}{u}=\frac{1}{f} \quad \Rightarrow \frac{1}{-u}-\frac{1}{u}=\frac{1}{20}$\ $\Rightarrow \frac{-2}{u}=\frac{1}{20} \Rightarrow u=-40 \mathrm{~cm}$\