A double convex lens is made of glass of refractive index `1.55` with both faces of same radius of curvature. Find the radius of curvature required, if focal length is `20 cm`.
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Here, `mu = 1.55, R_(1) = R and R_(2) = - R, f = 20 cm`. As `(1)/(f) = (mu - 1) ((1)/(R _(1)) - (1)/(R_(2))) :. (1)/(20) = (1.55 - 1)((1)/(R ) + (1)/(R )) = (1.10)/(R )` `R = 20 xx 1.1 = 2.2 cm`
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