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JAMB Physics 2019 Paper

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Question : 16

Total: 100

The distance between an object and its real image in a convex lens is 40cm. If the magnification of the image is 3, calculate the focal length of the lens

6.5 cm
7.5 cm
8.5 cm
4.5 cm

Solution:

u + v = 40

$\frac{v}{u}$ = 3

v = 3u

u + 3u = 40

4u = 40

u = 10cm

v = 3u = 30cm

f = $\frac{uv}{u+v} = \frac{10(30)}{10+30} = \frac{300}{40}$

= 7.5 cm

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