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The focal length of a convex lens is 20 cm. What is the power of this lens?
A+0.5 D
B+20 D
C+50 D
D+5 D
Answer & Solution
Correct answer: D. +5 D
1. Convert focal length to metres: f = 20 cm = 0.2 m, positive for a convex lens.
2. Power $P = \frac{1}{f(m)} = \frac{1}{0.2}$.
3. So P = +5 D; forgetting to convert cm to m gives the wrong +0.5 D value.
_Source: Balbharati (Maharashtra Board) Class 10 Science & Technology, Ch 7 "Lenses", p.94_
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