Question 281: Ray Optics

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 __________ $\mathrm{cm}$.

Accepted Answer

$\begin{aligned} & \mathrm{v}=3 \mathrm{u} \\ & \mathrm{v}-\mathrm{u}=20 \mathrm{~cm} \\ & 2 \mathrm{u}=20 \mathrm{~cm} \\ & \mathrm{u}=10 \mathrm{~cm}\end{aligned}$
$\begin{aligned} & \frac{1}{(-30)}-\frac{1}{(-10)}=\frac{1}{f} \\ & f=15 \mathrm{~cm}\end{aligned}$

Ray Optics

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}.

Choose the correct answer: