The power of lenses

The power of lens is a measure of its ability to converge or diverge an incident beam of light.
It is defined as the reciprocal of the focal length, in meters .
The unit of power is m^-1 or dioptre.

Optical power (also referred to as dioptric power, refractive power, focusing power, or convergence power)
Sourse : http://en.wikipedia.org/wiki/Lens_power

Worked example :
Determine the power of each of the following lenses.
a) Convex lens, f = 10 cm,
b) Concave lens, f = 10 cm.

Solution :
a) power,P = 1/0.1
= 10 D

b) power , P = -1/0.1
= -10 D