NEET Sample Paper NEET Sample Test Paper-32

  • question_answer 16) A lens of refractive index n is put in a liquid of refractive Index n' of focal length of lens in air is f, its focal length in liquid will be :-

    A)  \[-\frac{f\,n'(n-1)}{n'-n}\]

    B)  \[-\frac{f(\,n'-n)}{n'(n-1)}\]

    C)  \[-\frac{n'(\,n-1)}{f\,(n'-n)}\]

    D)  \[\frac{fn'n}{n-n'}\]

    Correct Answer: A

    Solution :

    \[\frac{1}{f}=\left( \frac{n-1}{1} \right)\,\left( \frac{1}{{{R}_{1}}}-\frac{1}{{{R}_{2}}} \right)\]and \[\frac{1}{f'}=\left( \frac{n-n'}{n'} \right)\,\left( \frac{1}{{{R}_{1}}}-\frac{1}{{{R}_{2}}} \right)\] \[\therefore \]     \[\frac{f'}{f}=\frac{n-1}{1}\times \frac{n'}{n-n'}\] \[\Rightarrow \]\[f=\frac{fn'(n-1)}{n'-n}\]

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