Physics · Geometrical Optics

JEE Main 2025 — 24 January, Morning Shift — Question 48

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

Answer: A

When $\mathrm{P}=2.5 \mathrm{D}$

$\mathrm{F}=\frac{1}{\mathrm{P}}=\frac{1}{2.5}$

When $\mathrm{P}^{\prime}=2.6 \mathrm{D}$

$\mathrm{F}^{\prime}=\frac{1}{\mathrm{P}^{\prime}}=\frac{1}{2.6}$

Relative decrease in focal length

$\frac{\mathrm{F}-\mathrm{F}^{\prime}}{\mathrm{F}}=\frac{\frac{2}{5}-\frac{5}{13}}{\frac{2}{5}}=1-\frac{25}{26}=\frac{1}{26}=0.04$

Answer key and solution verified before publishing.

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