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BHU PMT BHU PMT (Screening) Solved Paper-2010

  • question_answer
    The radii of curvature of the two surfaces of a lens are 20 cm and 30 cm and the refractive index of the material of the lens is 1.5. If the lens is concavo-convex, then the focal length of the lens is

    A)  24cm                                   

    B)  10cm

    C)  15cm                                   

    D)  120cm

    Correct Answer: D

    Solution :

                     The focal length of the lens                                                    \[\frac{1}{f}=(\mu -1)\left( \frac{1}{{{R}_{1}}}-\frac{1}{{{R}_{2}}} \right)\]                                                                   \[\frac{1}{f}=(1.5-1)\left( \frac{1}{20}-\frac{1}{30} \right)\]                                                                             \[\frac{1}{f}=0.5\left( \frac{30-20}{600} \right)\] Or                                                                          \[\frac{1}{f}=\frac{1}{2}\times \frac{10}{600}=\frac{1}{120}\] or                                                                                                               \[f=120\,cm\]


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