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Two lenses of powers $-1.75 \mathrm{D}$ and $+2.25 \mathrm{D}$ are placed in contact. The focal length of the combination is
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The correct answer is:
$200 \mathrm{~cm}$
$\mathrm{P}=\mathrm{P}_1+\mathrm{P}_2=-1.75+2.25=0.5 \mathrm{D}$
$\begin{aligned} & \Rightarrow \frac{1}{\mathrm{f}_{\text {net }}}=0.5 \\ & \Rightarrow \mathrm{f}_{\text {net }}=2 \mathrm{~m}=200 \mathrm{~cm}\end{aligned}$
$\begin{aligned} & \Rightarrow \frac{1}{\mathrm{f}_{\text {net }}}=0.5 \\ & \Rightarrow \mathrm{f}_{\text {net }}=2 \mathrm{~m}=200 \mathrm{~cm}\end{aligned}$
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