alleryStart learning →What are the signs of the focal length and power for a concave mirror and for a convex mirror?
AConcave negative, convex positive
BBoth concave and convex positive
CConcave positive, convex negative
DBoth concave and convex negative
Answer & Solution
Correct answer: C. Concave positive, convex negative
1. The focal length of a concave mirror is positive, because a concave mirror converges the rays that strike it.
2. The focal length and power of a convex mirror are negative, because a convex mirror diverges them.
3. Power is P = 1 divided by f for a mirror just as for a lens, so power carries the same sign as the focal length.
4. Getting these signs the wrong way round flips every later conclusion about whether the image is real or virtual.
_Source: OpenStax College Physics (CC BY 4.0), Ch 25 "Geometric Optics", section 25.7 Image Formation by Mirrors_
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