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BVP Medical BVP Medical Solved Paper-2003

  • 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: \[{{f}_{1}}=60cm\,{{f}_{2}}=-20cm\]) \[\frac{1}{f}=\frac{1}{60}-\frac{1}{20}=\frac{1-3}{60}=\frac{-2}{60}\] \[f=-30cm\]


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