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WB JEE Medical WB JEE Medical Solved Paper-2010

  • question_answer
    A plano-convex lens \[(f=20\,cm)\] is silvered at plane surface. Now focal length will be

    A)  20 cm                                  

    B)  40 cm

    C)  30 cm                                  

    D)  10 cm

    Correct Answer: D

    Solution :

                     The focal length of the plano-convex lens is 20 cm with the plane surface being silvered. So, the net power of the lens is given by \[P=\frac{1}{f}\times 2=\frac{2}{f}\] Thus, the net focal length is \[\frac{f}{2}=\frac{20}{2}=10\,cm\]


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