alleryAn object is placed 30 cm in front of a concave mirror of focal length 10 cm. The image distance is:
A+15 cm
B-7.5 cm
C-15 cm
D-30 cm
Answer & Solution
Correct answer: C. -15 cm
1. Apply 1/v + 1/u = 1/f with u = -30 cm, f = -10 cm.
2. 1/v = 1/f - 1/u = -1/10 + 1/30 = -3/30 + 1/30 = -2/30.
3. Hence v = -15 cm: the image is real, 15 cm in front of the mirror.
_Source: syllabus Science, Ch 9 "Light Reflection and Refraction", §9.2.4_
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