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Power of lens: The degree of convetgence or divergence of light rays that can achevied by a lens is expressed in terms of its power. The reciprocal of focal length is power of lens. Powe of lens P = 1/f( in m). P= 100/f (in cm). The unit of power of lens is dioptre. Denoted by D. NOW FINDING FOCAL LENGTH WHEN POWER OF LENTHVIS GIVEN. Example: doctor advised to use 2D lens. Find focal length. Sol: using, p = 100/f ( in cm); 2=100/f. Therefore, f= 100/2 =50 cm. The lens has focal length , f= 50 cm
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Power(P) = 1/Focal length(f) 

so Power is the reciprocal of the focal length of the lens

so you take out the Focal length

P = 1/f
f = 1/P

the unit is in metres

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