Question

A convex lens is used to obtain a magnified image of an object on a screen 10 cm from the lens. If the magnification is 19, find the focallength of the lens.

Answer 1

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Answer 2

Given that,

Magnification, m = 19

A convex lens is used to obtain a magnified image of an object on a screen 10 cm from the lens, image distance, v = -10 cm

To find : Focal length of the lens.

Solution,

Lens formula is :

$\dfrac{1}{v}-\dfrac{1}{u}=\dfrac{1}{f}$ .....(1)

Magnification,

$m=\dfrac{v}{u}\\\\\dfrac{v}{u}=-19\\\\u=\dfrac{-10}{19}\ m$

$\dfrac{1}{v}-\dfrac{1}{u}=\dfrac{1}{f}\\\\\dfrac{1}{f}=\dfrac{1}{10}+\dfrac{19}{10}\\\\f=0.5\ cm$

So, the focal length of the lens is 0.5 cm.

Learn more,

Lens formula

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