Question
The distance between object and its $3$ times magnified virtual image as produced by a convex lens is $20 \mathrm{~cm}$. The focal length of the lens used is $\mathrm{cm}$
✓
Answer
c
$\mathrm{v}=3 \mathrm{u}$
$\mathrm{v}=3 \mathrm{u}$
$\mathrm{v}-\mathrm{u}=20 \mathrm{~cm}$
$2 \mathrm{u}=20 \mathrm{~cm}$
$\mathrm{u}=10 \mathrm{~cm}$
$\frac{1}{(-30)}-\frac{1}{(-10)}=\frac{1}{\mathrm{f}}$
$\mathrm{f}=15 \mathrm{~cm}$
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