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(a) What is meant by ‘power of a lens’? (b) State and define the S.I. unit of power of a lens. (c) A convex lens of focal length 25 cm and a concave lens of focal length 10 cm are placed in close contact with each other. Calculate the lens power of this combination. |
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Answer» (a) Power of a Lens: The ability of a lens, to converge or diverge the ray of light after refraction, is called power (P) of the lens. It is defined as the reciprocal of the focal length, i.e. p = 1/f (b) The SI unit of power of a lens is ‘dioptre’. A lens of focal length 100 cm has a power of 1 dioptre, i.e. 1 dioptre = 1m–1. (c) Power of the combination p = p1 + p2 = (100/25 +100/-10) = 4-10 = -60 |
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