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Light Reflection and Refraction
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02/12/2020 4:38 pm
Topic starter
(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.
1 Answer
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02/12/2020 4:45 pm
(a) The ability of a lens to converge or diverge the ray of light after refraction is called power (P) of the lens. Its 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 = 1 m-1.
(c) Power of the combination
= P = P1 + P2
= 100/25 + 100/-10
= 4 - 10
= -6 D