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A convex lens and a concave lens, each having same focal length of $25 \mathrm{~cm}$, are put in contact to form a combination of lenses. The power in diopters of the combination is:
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The correct answer is:
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Focal length of lens
$$
f_1=25 \mathrm{~cm}
$$
focal length of concave lens
$$
f_2=25 \mathrm{~cm}
$$
Power of combination in dioptres
$$
\begin{aligned}
P=P_1+P_2 & =\frac{100}{f_1}+\frac{100}{f_2} \\
& =\frac{100}{25}+\frac{100}{-25}=0 .
\end{aligned}
$$
$$
f_1=25 \mathrm{~cm}
$$
focal length of concave lens
$$
f_2=25 \mathrm{~cm}
$$
Power of combination in dioptres
$$
\begin{aligned}
P=P_1+P_2 & =\frac{100}{f_1}+\frac{100}{f_2} \\
& =\frac{100}{25}+\frac{100}{-25}=0 .
\end{aligned}
$$
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