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"We know that S.I unit of power lens 1dioptre,D= reciprocal of focal length(f) in metre\r\nSo, D=1/f \r\nor, f=1/D=1/2= 0.5 metre= 50 c.m.\r\nSo, focal length of this lens is = 50 c.m."
<https://www.learnpick.in/local-tutors/delhi/all/physics/southern+avenue-area>