A glass convex lens is of refractive index 1.55 with both faces of same radius of curvature. What is the radius of curvature required if focal length is to be 20 cm ?

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Using the Lens maker's formula, ​ f 1 ​ = ( μ − 1 ) ( R 1 ​ 1 ​ − R 2 ​ 1 ​ ) Here, f = 20 cm , μ 1.55 , R 1 ​ = R , R 2 ​ = − R ⇒ 20 1 ​ = ( 1.55 − 1 ) ( R 1 ​ − ( − R ) 1 ​ ) ⇒ 20 1 ​ = 0.55 × R 2 ​ ⇒ R = 1.1 × 20 = 22 cm ​

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