alleryA convex lens of focal length 20 cm has an object placed at 30 cm from it. The image distance is:
A60 cm
B30 cm
C20 cm
D12 cm
Answer & Solution
Correct answer: A. 60 cm
1. Lens formula: 1/v − 1/u = 1/f. u = −30 cm, f = +20 cm.
2. 1/v = 1/f + 1/u = 1/20 + 1/(−30) = (3 − 2) / 60 = 1/60.
3. v = +60 cm — real image on the opposite side, magnified (m = v/u = 60/(−30) = −2).
4. So image is 60 cm from lens, inverted and twice as tall.
_Source: syllabus Science Ch 9 §9.3.6 (worked-example methodology)_
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