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A concave mirror has a radius of curvature of 0.8 m. Using f equals R over 2, what is its focal length?
A-0.8 m
B-0.4 m
C1.6 m
D0.4 m
Answer & Solution
Correct answer: D. 0.4 m
1. The focal length is half the radius of curvature: f = R/2 = 0.8 m / 2 = 0.4 m.
2. The mirror is concave, so f is positive under the stated sign convention.
3. 1.6 m doubles R instead of halving it, and either negative option wrongly applies the convex sign to a concave mirror.
_Source: OpenStax Physics (CC BY 4.0), Ch 16 "Mirrors and Lenses", section 16.1 Reflection_
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