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JEE Main & Advanced Sample Paper JEE Main Sample Paper-11

  • question_answer
    A thin convex lens of focal length 10cm and refractive index 1.5 is cut vertically into two equal pieces. They are placed as shown with a liquid of refractive index 3 between them. What is the focal length of the combination?

    A)  -10 cm.               

    B)  -10/4 cm.

    C)  -10/3 cm.          

    D)  None

    Correct Answer: C

    Solution :

     \[\frac{1}{f}=(\mu -1)\left( \frac{1}{{{R}_{1}}}-\frac{1}{R\to } \right)\]\[R=10\,cm.\] \[f'=(3-1)\left( \frac{1}{-10}-\frac{1}{10} \right)\]              \[=-\frac{10}{4}\] \[\frac{1}{{{f}_{eq}}}=\frac{1}{20}-\frac{4}{10}+\frac{1}{20}=\frac{2}{20}-\frac{4}{10}\] \[\Rightarrow {{f}_{eq}}=-\frac{10}{3}cm\]


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