The relationship between the object distance (u), the image distance (v) and the focal length (f) of the lens is called lens formula.
From the above formula, Focal length of a lens is obtained as follows:
The power of a lens is the measure of its ability to produce convergence or divergence of a parallel beam of light. The power of a lens depends on its focal length. The power of a lens is defined as the reciprocal of its focal length in metres. The unit of power of a lens is dioptre (D).
If two lenses of focal length f1 and f2 are in contact, then:
P = P1 +P2
For example, if a lens has a focal length of 40 cm, its power would be: 1/40 x10-2= 100/4 = 2.5D