alleryA convex lens of focal length 20 cm produces a real image at distance 60 cm. The object is at:
A60 cm in front of the lens
B20 cm in front of the lens
C30 cm in front of the lens
D15 cm in front of the lens
Answer & Solution
Correct answer: C. 30 cm in front of the lens
1. Use lens formula 1/v - 1/u = 1/f (note sign convention for lenses).
2. v = +60 cm, f = +20 cm. So 1/u = 1/v - 1/f = 1/60 - 1/20 = (1-3)/60 = -2/60.
3. u = -30 cm, i.e. 30 cm in front of the lens.
_Source: syllabus Science, Ch 9 "Light Reflection and Refraction", §9.4.4_
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