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RAJASTHAN PMT Rajasthan - PMT Solved Paper-1997

  • question_answer
    Focal length of a convergent lens is R. Now if lens is dipped into water of refractive index 1.33, then focal length of lens will be (refractive index of material of lens = 1.5):

    A)  R                           

    B)         2R                         

    C)  4R                         

    D)         R/2

    Correct Answer: C

    Solution :

    Ratio of focal lengths of lens in liquid and air.  \[\frac{{{f}_{1}}}{{{f}_{a}}}=\frac{{{(}_{a}}{{n}_{g}}-1)}{\left( \frac{_{a}{{n}_{g}}}{_{a}{{n}_{1}}}-1 \right)}\] \[\frac{{{f}_{1}}}{R}=\left( \frac{(1.5-1)}{\frac{1.5}{1.33}}-1 \right)\] \[{{f}_{1}}=4R\]            .


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