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RAJASTHAN PMT Rajasthan - PMT Solved Paper-1999

  • question_answer
    If focal length of convex lens is 9 cm and that of a concave lens is - 18 cm, then focal length of combination is:

    A)  + 9 cm                                 

    B)  -18 cm

    C)  -9 cm                   

    D)         +18 cm

    Correct Answer: D

    Solution :

    Focal length of combination \[\frac{1}{F}=\frac{1}{{{F}_{1}}}+\frac{1}{{{F}_{2}}}\] \[=\frac{1}{9}-\frac{1}{18}=\frac{2-1}{18}=\frac{1}{18}\] \[F=+10\,cm\]


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