CBSE 2025 · Region 2 · Set 2 · Q19 · 2 marks
Two convex lenses A and B, each of focal length $\displaystyle 10.0$ cm , are mounted on an optical bench at $\displaystyle 50.0$ cm and $\displaystyle 70.0$ cm respectively. An object is mounted at $\displaystyle 20.0$ cm . Find the nature and position of the final image formed by the combination.
Marking-scheme solution
For the first lens:
$\displaystyle \frac{1}{v_{1}}-\frac{1}{u_{1}}=\frac{1}{f_{1}}$
$\displaystyle \frac{1}{v_{1}}+\frac{1}{30}=\frac{1}{10}$
$\displaystyle v_{1}=15 \mathrm{~cm}$
For the second lens:
$\displaystyle u_{2}=-20-(-15)=-5 \mathrm{~cm}$
$\displaystyle \frac{1}{v_{2}}-\left(-\frac{1}{5}\right)=\frac{1}{10}$
On solving:
$\displaystyle v_{2}=-10 \mathrm{~cm}$
Image is virtual.
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