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AMU Medical AMU Solved Paper-2001

  • question_answer
    A drop of water is placed on a glass plate. A double convex lens having radius of curvature of each surface 20 cm is placed on it. The focal length of water lens \[\left( \mu =\frac{4}{3} \right),\] will be

    A)  0.20 m                                 

    B)  -0.60 m

    C)  0.30 m                                 

    D)  0.40 m

    Correct Answer: B

    Solution :

                     From lens formula                 \[\frac{1}{f}=(\mu -1)\left( \frac{1}{{{R}_{1}}}-\frac{1}{\sqrt{{{R}_{2}}}} \right)\] where \[{{R}_{1}},{{R}_{2}}\] are radii of curvatures and \[\mu \]the refractive index. Given,   \[R=-20\,cm\], \[{{R}_{2}}=\infty \] (plano-convex lens),                 \[\mu =\frac{4}{3}\]                 \[\frac{1}{f}=\left( \frac{4}{3}-1 \right)\left( \frac{1}{-20}-\frac{1}{\infty } \right)\] \[\Rightarrow \]               \[\frac{1}{f}=\frac{1}{3}\times \left( \frac{1}{-20} \right)\] \[\Rightarrow \]               \[f=-60\,\,cm=-0.6\,m\]


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