Two thin convex lenses of focal lengths 30 cm and 10 cm are placed coaxially 10 cm apart. The power of the combination is:
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Question

Two thin convex lenses of focal lengths 30 cm and 10 cm are placed coaxially 10 cm apart. The power of the combination is:

G1 Physics · Accepted Answer

For two thin lenses separated by distance d, the equivalent focal length satisfies 1/ f e q f_eq f e ​ q = 1/f₁ + 1/f₂ - d/(f₁f₂). Using SI units: f₁ = 0.30 m, f₂ = 0.10 m, d = 0.10 m. Therefore, 1/ f e q f_eq f e ​ q = 1/0.30 + 1/0.10 - 0.10/(0.30×0.10) = 3.333 + 10 - 3.333 = 10 m⁻¹. Hence the power is P = 1/ f e q f_eq f e ​ q = 10 D. Correct option: D.