alleryA concave mirror has a radius of curvature of 20 cm. Its focal length is:
A10 cm
B20 cm
C40 cm
D5 cm
Answer & Solution
Correct answer: A. 10 cm
1. Use R = 2f, so f = R/2.
2. Substitute R = 20 cm to get f = 10 cm.
3. The principal focus is 10 cm in front of the pole.
_Source: syllabus Science, Ch 9 "Light Reflection and Refraction", §9.2_
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