alleryA concave mirror with focal length 20 cm forms an image at 30 cm in front of it. The object is at:
A50 cm
B20 cm
C15 cm
D60 cm
Answer & Solution
Correct answer: D. 60 cm
1. Use 1/v + 1/u = 1/f. v = -30 cm, f = -20 cm.
2. 1/u = 1/f - 1/v = -1/20 + 1/30 = -3/60 + 2/60 = -1/60.
3. Hence u = -60 cm — the object is 60 cm in front of the mirror.
_Source: syllabus Science, Ch 9 "Light Reflection and Refraction", §9.2.4_
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