The radii of curvature of the surfaces of a double convex lens are `20 cm and 40 cm` respectively, and its focal length is `20 cm`. What is the refractive index of the material of the lens ?
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Here, `R_(1) = 20 cm, R_(2) = -40 cm, f = 30 cm, mu = ?` As `(1)/(f) = (mu - 1)((1)/(R_(1)) - (1)/(R_(2)))` `:. (1)/(20) = (mu - 1)((1)/(20) + (1)/(40)) = (mu - 1)(3)/(40)` or `(mu -1) = (2)/(3)` or `mu = (2)/(3) + 1 = (5)/(3)`
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