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JAMIA MILLIA ISLAMIA Jamia Millia Islamia Solved Paper-2012

  • question_answer
        The radius of the convex surface of plan convex lens is 20 cm and the refractive index of the material of the lens is 1.5. The focal length is

    A)  30 cm                                  

    B)  50 cm

    C)  20 cm                                  

    D)  40 cm

    Correct Answer: D

    Solution :

                    Using the relation for focal length of planoconvex lens \[\frac{1}{f}=(\mu -1)\left( \frac{1}{{{R}_{1}}}-\frac{1}{{{R}_{2}}} \right)\] (Refractive index of material of lens                      \[\mu =1.5,{{R}_{1}}=20\,cm,{{R}_{2}}=\infty )\]                 \[\frac{1}{f}=(1.5-1)\left( \frac{1}{20}-\frac{1}{\infty } \right)\]                 \[\frac{1}{f}=\frac{1}{40}\]or \[f=40\,cm\]


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