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A doctor prescribes spectacles having a lens of power +1.5 D. What is the focal length of this lens?
A−0.67 m
B+1.50 m
C+0.15 m
D+0.67 m
Answer & Solution
Correct answer: D. +0.67 m
1. Power and focal length are related by $P = \frac{1}{f(m)}$, so $f = \frac{1}{P}$.
2. Here $f = \frac{1}{+1.5} = +0.67$ m.
3. The positive sign means a convex lens; the negative option would wrongly suggest a concave lens.
_Source: Balbharati (Maharashtra Board) Class 10 Science & Technology, Ch 7 "Lenses", p.101_
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