Two thin convex lenses of focal length 30 cm and 10 cm are placed coaxially, 10 cm apart. The power of this combination is :

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(D) 10 D

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\(\mathrm{f}_1=30 \mathrm{~cm}, \mathrm{f}_2=10 \mathrm{~cm}\)
\(\frac{1}{\mathrm{f}_{\mathrm{eq}}}=\frac{1}{\mathrm{f}_1}+\frac{1}{\mathrm{f}_2}-\frac{\mathrm{d}}{\mathrm{f}_1 \mathrm{f}_2}, \mathrm{~d}=\) distance between lens
\(\frac{1}{\mathrm{f}_{\mathrm{eq}}}=\frac{1}{0.3}+\frac{1}{0.1}-\frac{0.1}{(0.3)(0.1)}\)
\(\frac{1}{\mathrm{f}_{\mathrm{eq}}}=\frac{1}{0.1}\)
Power \(=\frac{1}{\mathrm{f}_{\mathrm{eq}}}=10 \mathrm{D}\)

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