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Physics · 2025 · 2 marks
Ray Optics and Optical InstrumentsRefraction at Spherical Surfaces and by Lenses2 marksApplyvery short answer
CBSE 2025 · Region 2 · Set 3 · Q19
A light beam converges at a point O. In the path of this beam, a concave lens of focal length 15 cm is placed at a distance of 10 cm before point O.…
A light beam converges at a point O . In the path of this beam, a concave lens of focal length $\displaystyle 15$ cm is placed at a distance of $\displaystyle 10$ cm before point O . The beam now converges at a point $\displaystyle \mathrm{O}^{\prime}$. Find the magnitude and the direction of shift $\displaystyle \mathrm{OO}^{\prime}$.
Marking-scheme solution
$\displaystyle \frac{1}{v}-\frac{1}{u}=\frac{1}{f}$
$\displaystyle \frac{1}{v}=-\frac{1}{15}+\frac{1}{10}$
On solving:
$\displaystyle v=30 \mathrm{~cm}$
Magnitude of $\displaystyle O O^{\prime}=30-10=20 \mathrm{~cm}$
Direction is away from the lens.
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