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Two lenses are placed in contact with each other and the focal length of combination is \mathrm{80\ cm}. If the focal length of one is \mathrm{20\ cm}, then the power of the other will be:

Option: 1

\mathrm{1.66\ D}


Option: 2

\mathrm{4.00\ D}


Option: 3

\mathrm{-1.00\ D}


Option: 4

\mathrm{-3.75\ D}


Answers (1)

best_answer

Focal length of the combination, \mathrm{\frac{1}{\mathrm{f}}=\frac{1}{\mathrm{f}_1}+\frac{1}{\mathrm{f}_2} \Rightarrow \frac{1}{80}=\frac{1}{20}+\frac{1}{\mathrm{f}_2}}
\mathrm{\therefore f_{2}=-\frac{80}{3}\ cm}
\mathrm{\therefore \quad \text { Power of second lens } P^{\prime}=\frac{100}{f_2}=\frac{100}{(-80 / 3)}=-3.75 \mathrm{D}}

Posted by

Ritika Jonwal

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