A) 50 cm
B) 30 cm
C) -50 cm
D) -30 cm
Correct Answer: D
Solution :
\[\text{Magnification = }\frac{\text{image distance}}{\text{object distance}}\] \[\therefore \] \[M=-\frac{v}{u}=2\] \[\Rightarrow \] \[v=-2u\] Also from lens formula for focal length \[f,v\]as image distance and u as object distance, we have \[\frac{1}{f}=\frac{1}{v}=\frac{1}{u}\] Putting \[f=20\,cm,\,v=-2u,\]we get \[\frac{1}{20}=-\frac{1}{2u}-\frac{1}{u}\] \[\Rightarrow \] \[\frac{1}{20}=-\frac{1-2}{2u}\] \[\Rightarrow \] \[u=-30\,cm\]
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