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The lens-maker's formula for a thin lens (refractive index n) with radii of curvature R₁ and R₂ is:

A1/f = (n − 1)(1/R₁ − 1/R₂)
B1/f = (n − 1)(1/R₁ + 1/R₂)
C1/f = n (R₁ + R₂)
D1/f = (n + 1)/R
Answer & Solution
Correct answer: A. 1/f = (n − 1)(1/R₁ − 1/R₂)
**Lens-maker's formula: 1/f = (n − 1)(1/R₁ − 1/R₂)** (thin lens in air; n = refractive index of lens material). Sign convention applies to R₁ and R₂. Used to design lenses or determine f from geometry/material.
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