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What is the relative decrease in focal length of a lens for an increase in optical power by 0.1D from 2.5D? ['D' stands for dioptre]

Asked in JEE Main 24th Jan 1st Shift 2025 · Lens maker's formula and power

Answer: (1) 0.04

Step-by-step solution

f=1/P, so (|Δ f|)/f=(Δ P)/P for a small change.

(Δ P)/P=(0.1)/(2.5)=0.04

Exact check: f goes from 0.400 m to 0.3846 m, a drop of 3.8%, about 0.04.

Why the other options are wrong

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