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Manipal Engineering Manipal Engineering Solved Paper-2013

  • question_answer
    The plano-convex lens of focal length 20 cm and 30 cm are placed together to form a double convex lens, the final focal length will be

    A)  12 cm 

    B)                         60cm  

    C)  20cm   

    D)         30cm

    Correct Answer: A

    Solution :

    Equivalent focal length                 \[\frac{1}{F}=\frac{1}{{{f}_{1}}}+\frac{1}{{{f}_{2}}}\]                      \[=\frac{1}{20}+\frac{1}{30}\]                 \[F=\frac{20\times 30}{20+30}=\frac{600}{60}=12\,\,cm\]                 \[F=12\,\,cm\]


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