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JEE Main & Advanced JEE Main Paper (Held On 15 April 2018) Slot-I

  • question_answer
    A plan convex lens becomes an optical system of \[28cm\] focal length when its plane surface is silvered and illuminated from left to right as shown in Fig-A. If the same lens is instead silvered on the curved surface and illuminated from other side as in Fig. B, it acts like an optical system of focal length\[10cm\]. The refractive index of the material of lens if:           [JEE Online 15-04-2018]

    A) \[1.50\]            

    B) \[1.55\]            

    C) \[1.75\]            

    D) \[1.51\]

    Correct Answer: B

    Solution :

    \[\frac{l}{F}=\frac{2}{f}+\frac{l}{{{f}_{m}}}\] \[{{f}_{m}}\Rightarrow \infty \] This gives, \[f=56\] \[{{f}_{m}}=\frac{R}{2}\backslash \] Hence \[\frac{1}{F}=\frac{2}{f}+\frac{2}{R}\] This gives, \[R=31.11\] \[\frac{1}{f}=(\mu -1)(\frac{1}{R})\] \[\mu =1.55\]


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