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Two converging lenses of focal length $20 \mathrm{~cm}$ and $40 \mathrm{~cm}$ are placed in contact. The effective power of the combination is
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$7.5 \mathrm{D}$
Given, $f_{1}=20 \mathrm{~cm}, f_{2}=40 \mathrm{~cm}$
Since, power $P=\frac{1}{f} \Rightarrow P_{1}=\frac{1}{f_{1}}=\frac{100}{20}=5 \mathrm{D}$ and $\quad P_{2}=\frac{1}{f_{2}}=\frac{100}{40}=25 \mathrm{D}$
Effective power, $P=P_{1}+P_{2}=5+25=7.5 \mathrm{D}$
Since, power $P=\frac{1}{f} \Rightarrow P_{1}=\frac{1}{f_{1}}=\frac{100}{20}=5 \mathrm{D}$ and $\quad P_{2}=\frac{1}{f_{2}}=\frac{100}{40}=25 \mathrm{D}$
Effective power, $P=P_{1}+P_{2}=5+25=7.5 \mathrm{D}$
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