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The image of a candle flame placed at a distance of 45 cm from a spherical lens is formed on a screen placed at a distance of 90 cm. from the lens identify the type of the lens and calculate the focal length. |
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Answer» Here u = -45 cm, v = +90 cm \(\frac{1}{f}=\frac{1}{v}-\frac{1}{u}\) = \(\frac{1}{90}-\frac{1}{-45}\) = \(\frac{1}{90}+\frac{1}{45}\) = \(\frac{1+2}{90}\) = \(\frac{3}{90}\) = \(\frac{1}{30}\) ∴ f = +30 cm As f is positive, the lens used is convex lens with focal length 30 cm. |
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