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BHU PMT BHU PMT (Mains) Solved Paper-2006

  • question_answer
    A convex lens is in contact with concave lens. The magnitude of the ratio of their focal lengths is 2/3. Their equivalent focal length is 30 cm. What are their individual focal lengths?

    A) \[-75,\text{ }50\]             

    B) \[-10,15\]

    C)  \[75,\text{ }50\]                             

    D) \[-15,10\]

    Correct Answer: D

    Solution :

                     Let focal length of convex lens is\[+f,\]then length of concave lens would be\[-\frac{3}{2}f\]. From the given condition                 \[\frac{1}{30}=\frac{1}{f}-\frac{2}{3f}=\frac{1}{3f}\] \[\therefore \]                  \[f=10\text{ }cm\] Therefore, focal length of convex lens\[=+10\text{ }cm\]and that of concave lens\[=-15\text{ }cm\].


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