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BHU PMT BHU PMT (Mains) Solved Paper-2008

  • question_answer
    A diverging meniscus lens of 1.5 refractive index has concave surfaces of radii 3 and 4 cm. The position of the image, if an object is placed 12 cm in front of the lens, is

    A)  7cm                                      

    B) \[-8cm\]

    C)   9cm                                     

    D)  10cm

    Correct Answer: B

    Solution :

                      \[\frac{1}{f}=(\mu -1)\left( \frac{1}{{{R}_{1}}}-\frac{1}{{{R}_{2}}} \right)\] For given concave lens,                 \[{{R}_{1}}=-3cm,\] \[{{R}_{2}}=-4cm\] \[\therefore \]  \[\frac{1}{v}-\frac{1}{u}=(\mu -1)\left( \frac{1}{-3}+\frac{1}{4} \right)\] Or           \[\frac{1}{v}-\frac{1}{(-12)}=(1.5-1)\left( \frac{-4+3}{12} \right)\]              Or           \[\frac{1}{v}+\frac{1}{12}=0.5\times \frac{-1}{12}=\frac{-1}{24}\] Or           \[\frac{1}{v}=-\frac{1}{24}-\frac{1}{12}\] \[=\frac{-1-2}{24}=-\frac{1}{8}\]                Or           \[v=-8\,cm\]


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