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UPSC NDA 2 2021 General Ability Solved Paper

Question : 132 of 150

Marks: +1, -0

Power of a lens of focal length 25 cm is

Solution:

Concept:

Power of a lens is the reciprocal of its focal length in meters. It measures the lens's ability to converge or diverge light.

Explanation:

Given focal length

f = 25\ \mathrm{cm}

Convert focal length to meters:

f = 0.25\ \mathrm{m}

Power

P = \frac{1}{f(\text{m})} = \frac{1}{0.25} = +4\ \mathrm{D}

Alternatively, use

P = \frac{100}{f(\text{cm})} = \frac{100}{25} = +4\ \mathrm{D}

The positive sign indicates a convex (converging) lens.

Answer:

+4 Diopter (Option C).

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