JEE MAIN - Physics (2024 - 1st February Morning Shift - No. 27)
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 __________ $\mathrm{cm}$.
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Giải thích
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$\begin{aligned} & \mathrm{v}=3 \mathrm{u} \\\\ & \mathrm{v}-\mathrm{u}=20 \mathrm{~cm} \\\\ & 2 \mathrm{u}=20 \mathrm{~cm} \\\\ & \mathrm{u}=10 \mathrm{~cm}\end{aligned}$
$\begin{aligned} & \frac{1}{(-30)}-\frac{1}{(-10)}=\frac{1}{f} \\\\ & f=15 \mathrm{~cm}\end{aligned}$