For the thin convex lens, the radii of curvature are at 15 cm and 30 cm respectively. The focal length the lens is 20 cm . The refractive index of the material is :

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​ f 1 ​ = ( μ air ​ μ lens ​ ​ − 1 ) ( R 1 ​ 1 ​ − R 2 ​ 1 ​ ) ⇒ + 20 1 ​ = ( 1 μ ​ − 1 ) ( + 15 1 ​ − ( − 30 ) 1 ​ ) ⇒ 20 1 ​ = ( μ − 1 ) ( 30 3 ​ ) ⇒ μ − 1 = 2 1 ​ ⇒ μ = 1 + 2 1 ​ = 2 3 ​ = 1 ⋅ 5 ​

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