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Light — Physics STD 10 — Question

ICSE BoardEnglish MediumSTD 10PhysicsLight2 Marks
Question
A convex lens forms a 2.5 times magnified virtual image when an object is placed at a distance of 8 cm from the lens. Calculate
(a) the distance of the image from the lens
(b) the focal length of lens.
✓

Answer

(a) Given, $m=2.5 ; u=-8 cm$ ( $\because u$ is always negative)
$m=\frac{V}{n} \Rightarrow V=m \times u=2.5 \times-8=-20 cm \quad(\because$ image is virtual)
(b) Again, $\frac{1}{f}=\frac{1}{V}-\frac{1}{u}=\frac{1}{-20}-\frac{1}{-8}=\frac{-2+5}{40}=\frac{3}{40}$
$\Rightarrow f-\frac{40}{3}=13.3 cm$

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