For a concave mirror, focal length f relates to radius of curvature R as:
Af = R/2
Bf = 2R
Cf = -R
Df = R
Answer & Solution
Correct answer: A. f = R/2
For spherical mirrors (small aperture): f = R/2. Concave (converging) mirror: positive f. Convex (diverging): negative f by convention. Holds for paraxial rays.
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