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Physics · 2024 · 2 marks
Ray Optics and Optical InstrumentsRefraction at Spherical Surfaces and by Lenses2 marksApplyvery short answer
CBSE 2024 · Region 5 · Set 3 · Q19
A thin converging lens of focal length 10 cm is placed coaxially in contact with a thin diverging lens of focal length 15 cm. How will the…
A thin converging lens of focal length $\displaystyle 10$ cm is placed coaxially in contact with a thin diverging lens of focal length $\displaystyle 15$ cm . How will the combination behave? Justify your answer.
Marking-scheme solution
It will behave like a converging lens.
Power of the converging lens is more than the power of the diverging lens. Hence the combination will behave like a converging lens.
OR
$\displaystyle P=P_{1}+P_{2}$
$\displaystyle =\frac{100}{10}+\frac{100}{-15}$
$\displaystyle P=\frac{10}{3} \mathrm{D}$
OR
$\displaystyle \frac{1}{f}=\frac{1}{f_{1}}+\frac{1}{f_{2}}$
$\displaystyle \frac{1}{f}=\frac{1}{10}-\frac{1}{15}$
$\displaystyle \frac{1}{f}=\frac{1}{30}$
$\displaystyle f=30 \mathrm{~cm}$
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