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VMMC VMMC Medical Solved Paper-2005

  • question_answer
    Two thin lenses, one of focal length + 60 cm and the other of focal length -20 cm are put in contact, the combined focal length is:

    A) 15 cm                   

    B)        -15 cm  

    C)        -30 cm                  

    D)        30 cm

    Correct Answer: C

    Solution :

    Using the relation for combined focal length \[\frac{1}{f}=\frac{1}{{{f}_{1}}}+\frac{1}{{{f}_{2}}}=\frac{1}{60}+\frac{1}{(-20)}\] (Here: \[{{f}_{1}}=60\,cm\,{{f}_{2}}=-20\,cm\]) \[\frac{1}{f}=\frac{1}{60}-\frac{1}{20}=\frac{1-3}{60}=\frac{-2}{60}\] \[f=-30\,cm\]


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