A plano-convex lens is made of material having refractive index 1.5. The radius of curvature of the curved surface is 60 cm. The focal length of the lens is cm.

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Accepted Answer

(2) 120. Given: plano-convex lens, n = 1.5, curved face of radius 60 cm, the other face plane. Idea: lens maker's formula 1/f = (n-1)(1/(R₁)-1/(R₂)), with R = ∞ for a plane face. Take the curved face first: R₁ = +60 cm and R₂ = ∞, so 1/(R₂) = 0. 1/f = (1.5-1)(1/(60)-0) = (0.5)/(60). f = (60)/(0.5) = 120 cm, and the positive sign confirms it converges. Turning the lens round gives the same answer, so the focal length is 120 cm.

A plano-convex lens is made of material having refractive index 1.5. The radius of curvature of the curved surface is 60 cm. The focal length of the lens is ______ cm.

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