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Light – Reflection and Refraction — Science STD 10 — Question

CBSE BoardEnglish MediumSTD 10ScienceLight – Reflection and Refraction2 Marks
Question
A lens has power 4D. Find the focal length that lens.
✓

Answer

Power of a lens is determined by the formula

$\text{P}=\frac{1}{\text{F}}$ (when focal length is in meters) and

$\text{P}=\frac{100}{\text{F}}$ (when focal length is in centimeters).

Here, the power is given as 4 diopters. On equating we get,

$4=\frac{100}{\text{F}}$

$\text{F}=\frac{100}{4}$

$=25\text{cm}$

Hence the focal length of lens is 25cm.

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