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Sunlight is focused by a magnifying glass to a bright spot 8.00 cm from the lens. Using P equals 1 over f, what is the power of this lens?
A0.08 D
B1.25 D
C8.00 D
D12.5 D
Answer & Solution
Correct answer: D. 12.5 D
1. Since the Sun's rays are nearly parallel, the bright spot forms at the focal point, so f = 8.00 cm.
2. Converting to metres for the power equation: f = 0.0800 m.
3. Power is P = 1/f = 1 / 0.0800 m.
4. Carrying out the division gives P = 12.5 D.
5. 0.08 D and 8.00 D both confuse the focal length itself, in different units, with the reciprocal that power actually requires.
_Source: OpenStax Physics (CC BY 4.0), Ch 16 "Mirrors and Lenses", section 16.3 Lenses_
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