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JEE Main & Advanced Physics Question Bank Human Eye and Lens Camera

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    A student can distinctly see the object upto a distance 15 cm. He wants to see the black board at a distance of 3 m. Focal length and power of lens used respectively will be  [Pb. PMT 2003]

    A)            \[-\,4.8\,cm,\ -\,3.3\,D\] 

    B)            \[-\,5.8\,cm,\ -\,4.3\,D\]

    C)            \[-\,7.5\,cm,\ -\,6.3\,D\] 

    D)            \[-\,15.8\,cm,\ -\,6.3\,D\]

    Correct Answer: D

    Solution :

                       \[v=-15cm,\] \[u=-\,300cm,\] From lens formula \[\frac{1}{f}=\frac{1}{v}-\frac{1}{u}\] \[\Rightarrow \frac{1}{f}=\frac{1}{-15}-\frac{1}{-300}=\frac{-19}{300}\]\[\Rightarrow f=\frac{-\,300}{19}=-\,15.8\,cm\] and power \[P=\frac{100}{f}cm\]\[=\frac{-100\times 19}{300}\]= ­? 6.33 D.


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