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Light Reflection and Refraction
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18/08/2021 12:14 pm
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The filament of a lamp is 80 cm from a screen and a converging lens forms an image of it on a screen, magnified three times. Find the distance of the lens from the filament and the focal length of the lens.
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18/08/2021 12:18 pm
-u + v = 80 cm ......(1)
m = -3 (The image is real, since it forms on a screen)
m = \(\frac{v}{u}\) = -3
v = -3u
Put in eq (1),
-u - 3u = 80
-4u = 80
u = -20 cm
Distance of lens from filament is 20 cm.
v = -3u = 60 cm
\(\frac{1}{v}\) - \(\frac{1}{u}\) = \(\frac{1}{f}\)
\(\frac{1}{60}\) - \(\frac{1}{-20}\) = \(\frac{1}{f}\)
\(\frac{1}{f}\) = \(\frac{1 + 3}{60}\) = \(\frac{4}{60}\)
f = 15 cm