Double convex lenses are to be manufactured from a glass of refractive index 1.55, with both faces of same radius of curvature. What is the radius of curvature required if the focal length is to be 20 cm?
A
11 cm
B
22 cm
C
7 cm
D
6 cm
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Solution
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Lens maker's formula, 1f=(μ−1)(1R1−1R2) Here, f = 20 cm, μ = 1.55, R1 = R, R2 = -R 120=(1.55−1)(1R−1(−R))or120=0.55×2R ⇒R=1.1×20=22cm
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