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NEET Sample Paper NEET Sample Test Paper-22

  • question_answer
    A plano-convex lens acts like a concave mirror of 28 cm focal length when its plane surface is silvered and like a concave mirror of 10 cm focal length when its curved surface is silvered. What is the refractive index of the material of the lens?

    A)  1.50                           

    B)  1.55   

    C)       1.60                           

    D)  1.65               

    Correct Answer: B

    Solution :

    In the first case we have\[\frac{1}{28}=\frac{2}{{{f}_{1}}}+\frac{1}{{{f}_{m}}}=\frac{2}{{{f}_{1}}}\] In the second case we have \[\frac{1}{10}=\frac{2}{{{f}_{1}}}+\frac{1}{{{f}_{m}}}\]where \[{{f}_{m}}\]is the focal length of silvered surface. By subtraction:\[\frac{1}{{{f}_{m}}}=\frac{1}{10}=\frac{1}{28}\] or, \[R=2{{f}_{m}}=\frac{280}{9}\,cm.\]\[\therefore \]\[{{f}_{1}}=28\times 2=56\,cm\] \[\frac{1}{{{f}_{1}}}=(\mu -1).\frac{1}{R}\]or \[\mu -1=\frac{R}{{{f}_{1}}}\]or \[\mu =1.55\] Hence, the correction option is [b].


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