JEE Main & Advanced JEE Main Paper (Held on 09-4-2019 Afternoon)

  • question_answer
    A thin convex lens L (refractive index = 1.5) is placed on a plane mirror M. When a pin is placed at A, such that OA = 18 cm, its real inverted image is formed at A itself, as shown in figure. When a liquid of refractive index \[{{\mu }_{1}}\] is put between the lens and the mirror, The pin has to be moved to A', such that OA' = 27 cm, to get its inverted real image at A' itself. The value of μ1 will be :-             [JEE Main 9-4-2019 Afternoon]

    A) \[\sqrt{2}\]                              

    B) \[\frac{4}{3}\]

    C) \[\sqrt{3}\]                              

    D) \[\frac{3}{2}\]

    Correct Answer: B

    Solution :

    \[\frac{1}{{{f}_{1}}}=\frac{1}{2}\times \frac{2}{18}=\frac{1}{18}\] \[\frac{1}{{{f}_{2}}}=\frac{({{\mu }_{1}}-1)}{-18}\] when\[{{\mu }_{1}}\]is filled between lens and mirror \[P=\frac{2}{18}-\frac{2}{18}({{\mu }_{1}}-1)=\frac{2-2{{\mu }_{1}}+2}{18}\] \[={{F}_{m}}=-\left( \frac{18}{2-{{\mu }_{1}}} \right)\] \[2=6-3{{\mu }_{1}}\] \[3{{\mu }_{1}}=4\] \[{{\mu }_{1}}=4/3\].


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