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The power of a lens is $-4\ \text{D}$. What is its focal length?
A$-0.4\ \text{m}$
B$-0.25\ \text{m}$
C$-4\ \text{m}$
D$-2.5\ \text{m}$
Answer & Solution
Correct answer: B. $-0.25\ \text{m}$
1. Power and focal length are related by $P = \dfrac{1}{f}$, with $f$ in metres.
2. Rearrange: $f = \dfrac{1}{P}$.
3. Substitute: $f = \dfrac{1}{-4} = -0.25$ m.
4. The negative sign confirms it is a concave (diverging) lens.
5. The answer $-4$ m wrongly treats power as the focal length itself.
_Source: Samacheer Kalvi (TN SCERT) Class 10 Science, Unit 2 Optics "Power of a Lens", p.30_
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