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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?

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Solution

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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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