The ray diagram shows the image of an object formed by a converging lens. What is the focal length of the lens?

+50 cm

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+40 cm

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-40 cm

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-90 cm

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Solution

From lens formula,
$\frac{1}{f} = \frac{1}{v} - \frac{1}{u}$
Distance of the object, $u = - 92 c m$
Distance of the image, $v = 72 c m$
$\frac{1}{f} = \frac{1}{72} - \frac{1}{- 90}$
$\frac{1}{f} = \frac{5 + 4}{360} = \frac{9}{360}$
$f = \frac{360}{9} = 40 c m$
The focal length of the lens=40cm

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Figure
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The ray diagram shows the image of an object formed by a converging lens. What is the focal length of the lens?

From lens formula,1f=1v−1uDistance of the object,u=−92cm Distance of the image,v=72cm 1f=172−1−90 1f=5+4360=9360 f=3609=40cmThe focal length of the lens=40cm

From lens formula,
$\frac{1}{f} = \frac{1}{v} - \frac{1}{u}$
Distance of the object, $u = - 92 c m$
Distance of the image, $v = 72 c m$
$\frac{1}{f} = \frac{1}{72} - \frac{1}{- 90}$
$\frac{1}{f} = \frac{5 + 4}{360} = \frac{9}{360}$
$f = \frac{360}{9} = 40 c m$
The focal length of the lens=40cm