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J & K CET Engineering J and K - CET Engineering Solved Paper-2013

  • question_answer
    When an object is placed \[40\text{ }cm\]from a diverging lens, its virtual image is formed \[20\text{ }cm\]from the lens. The focal length and power of lens are

    A)  \[F=-20cm,\,P=-5\,D\]

    B)  \[F=-40cm,\,P=-5\,D\]

    C)  \[F=-40cm,\,P=-2.5\,D\]

    D)  \[F=-20\,cm,\,P=-2.5\,D\]

    Correct Answer: C

    Solution :

    \[\frac{1}{f}=\frac{1}{v}-\frac{1}{u}\] \[\frac{1}{f}=\frac{1}{-20}-\left( -\frac{1}{40} \right)=\frac{-2+1}{40}=-\frac{1}{40}\] \[f=-40\,cm\] \[P=-\frac{200}{0.40}=-2.5\,D\]


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