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