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The relation between focal length f and radius of curvature R for a spherical mirror is:
Af = R
Bf = 2R
Cf = R/2 (paraxial approximation)
Df = R/4
Answer & Solution
Correct answer: C. f = R/2 (paraxial approximation)
For a spherical mirror under the paraxial approximation, **f = R/2**. Concave (converging) has positive R/2 in magnitude; sign depends on convention (f negative in Cartesian convention for a concave mirror with its centre of curvature on the object side).
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