A plano-convex lens made of refractive index 1.5 and having radius of curvature R =4 ~cm fits exactly into a planoconcave lens made of refractive index 1.3 and having the same radius of curvature R=4 ~cm such that their plane surfaces are p

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A plano-convex lens made of refractive index 1.5 and having radius of curvature R =4 ~cm fits exactly into a planoconcave lens made of refractive index 1.3 and having the same radius of curvature R=4 ~cm such that their plane surfaces are parallel to each other. The focal length of the combination is:

Quantrex Academy · Accepted Answer

Correct answer: D. 20 cm

For the plano-convex lens ( ₁ = 1.5 ), R₁ = , R₂ = -4 cm: 1 f₁ = (1.5 - 1) ( 1 - 1 -4 ) = 0.5 4 cm ⁻¹ For the plano-concave lens ( ₂ = 1.3 ), R₁ = -4 cm, R₂ = : 1 f₂ = (1.3 - 1) ( 1 -4 - 0 ) = - 0.3 4 cm ⁻¹ 1 F = 0.5 4 - 0.3 4 = 0.2 4 = 1 20 cm ⁻¹ F = 20 cm

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