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BCECE Engineering BCECE Engineering Solved Paper-2010

  • question_answer
    The figure shows an equiconvex lens of focal length \[f.\] If the lens is cut along PQ, the focal length of each half will be

    A) \[\frac{f}{2}\]                                  

    B) \[f\]

    C) \[2f\]                   

    D) \[4f\]

    Correct Answer: C

    Solution :

    According to Lens Makers formula, \[\frac{1}{f}=(\mu -1)\left( \frac{1}{R}-\frac{1}{R} \right)\]                 \[\Rightarrow \]               \[\frac{1}{f}=(\mu -1)\left( \frac{2}{R} \right)\]                  (i)                 On cutting,\[\frac{1}{f}=(\mu -1)\left( \frac{1}{R}-\frac{1}{\infty } \right)\]                 \[\frac{1}{f}=(\mu -1)\left( \frac{1}{R} \right)\]                  (ii) From Eqs. (i) and (ii), we get \[f=2f\]


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