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Two co-axial lenses of power $+4 \mathrm{D}$ and $-2 \mathrm{D}$ are placed in contact. The focal length of combination is
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The correct answer is:
$0.5 \mathrm{~m}$
The focal length of combination of two lenses,
$\begin{aligned}
\frac{1}{f} &=\frac{1}{f_{1}}+\frac{1}{f_{2}}=\frac{1}{(1 / 4)}+\frac{1}{(-1 / 2)} \quad\left(f=\frac{1}{P}\right) \\
&=4-2=2 \\
\Rightarrow f &=0.5 \mathrm{~m}
\end{aligned}$
$\begin{aligned}
\frac{1}{f} &=\frac{1}{f_{1}}+\frac{1}{f_{2}}=\frac{1}{(1 / 4)}+\frac{1}{(-1 / 2)} \quad\left(f=\frac{1}{P}\right) \\
&=4-2=2 \\
\Rightarrow f &=0.5 \mathrm{~m}
\end{aligned}$
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