Question

    What is the focal length of a double convex lens for

    which the radius of curvature of each surface is 30 cm and the refractive index of glass is 2.50 ?
    A 12 cm Correct Answer Incorrect Answer
    B 10 cm Correct Answer Incorrect Answer
    C 20 cm Correct Answer Incorrect Answer
    D 30 cm Correct Answer Incorrect Answer

    Solution

    Here given Surface curvature (R ) = 30 cm., Refractive index (n) = 2.50 For a double convex lens, R = −1R1 1/f = (n-1) {1/(R1 - R2)} => 1/f = (2.5 - 1) {1/30 + 1/30} => 1/f = 1/10 => f = 10 cm Hence, the focal length of a double convex length is 10 cm.

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