JEE Main & Advanced JEE Main Paper Phase-I (Held on 07-1-2020 Evening)

  • question_answer
    A thin lens made of glass (refractive index = 1.5) of focal length f = 16 cm is immersed in a liquid of refractive index 1.42. If its focal length in liquid is \[{{f}_{l}}\], then the ratio \[\frac{{{f}_{l}}}{f}\]is closest to the integer [JEE MAIN Held on 07-01-2020 Evening]

    A) 17

    B) 1

    C) 5         

    D) 9

    Correct Answer: A

    Solution :

    [a] \[\frac{1}{{{f}_{a}}}=(1.5-1)\,\left[ \frac{1}{{{R}_{1}}}-\frac{1}{{{R}_{2}}} \right]\] Lens Maker?s formula  \[\frac{1}{{{f}_{e}}}=\left( \frac{1.5}{1.42}-1 \right)\,\left( \frac{1}{{{R}_{1}}}-\frac{1}{{{R}_{2}}} \right)\] \[\Rightarrow \,\,\frac{{{f}_{e}}}{{{f}_{a}}}=\frac{(1.5-1)1.42}{0.08}=\frac{1.42}{0.16}=\frac{142}{16}\,\approx 9\]    

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