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What is meant by power of a lens? Write its SI unit. A student uses a lens of focal length 40 cm and another of -20 cm . Write the nature and power of each lens. |
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Answer» Power of a lens. The power of a lens is a measure of the degree of convergence or divergence of light rays falling on it. SI unit of power of lens is dioptre (D). Given : `f = 40 cm = (40)/(100) m = 2/5 m` `P = 1/(f(m)) = 1/(2//5) = 5/2 = + 2.5 D` Nature of lens is convex lens. Given : `f = - 20 cm = 20/1000 m = (-1)/(5) m` `P = (1)/(f(m)) = (1)/(-1//5) = (-5)/(1) = -5 D` Nature of lens is concave lens. |
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