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JEE Main & Advanced Sample Paper JEE Main Sample Paper-35

  • question_answer
    A convex lens of focal length T cut into parts first horizontally and then vertically. Find the focal length of part A of the lens, as shown.             

    A)  \[f/2\]                                 

    B)  \[f\]

    C)  \[4f\]                                   

    D)  \[2f\]

    Correct Answer: D

    Solution :

    When cut horizontally             \[\frac{1}{f}\,=(\mu -1)\,\left[ \frac{1}{R}\,-\frac{1}{(-R)} \right]\,=\frac{2(\mu -1)}{R}\] Focal length remains same when cut vertically             \[\frac{1}{f'}\,=(\mu -1)\,\left[ \frac{1}{R}\,-\frac{1}{\infty } \right]\,=\frac{(\mu -1)}{R}\,=\frac{1}{2f}\] (from eq.(1)           \[f'=2f\]


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