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For a spherical convex mirror, how is the focal length f connected to its radius of curvature r?

1. f = +r/2
2. f = -r
3. f = -r/2
4. f = r

Correct answer: f = +r/2

Solution

The mirror relation is f = R/2. In the Cartesian sign convention the center of curvature of a convex mirror is behind it, so R is positive and the focal length is positive: f = +r/2.

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