The focal length of a concave mirror is 15 cm. The radius of curvature is
A. 7.5 cm
B. 15 cm
C. 45 cm
D. 30 cm
Answer: Option D
Solution (By JKSSB Mock Tests)
For spherical mirrors, R = 2f. So R = 2 × 15 = 30 cm. Sign convention: for concave mirror, f and R are negative by Cartesian convention, but magnitude 30 cm. So 30 cm is correct.
Explanation:
The principle of homogeneity states that you can only add or subtract physical quantities that have the exact same dimensional formula (you can't add mass to length). Therefore, in the equation A = B + C, the terms B and C must have the same dimensions, and their sum will yield a quantity A that also shares those exact same dimensions.
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