Theory:

Relation between the power of lens and focal length
The ability of a ray of light to converge or diverge when it strikes a lens is determined by the focal length of the lens. The power of a lens refers to its ability to converge (convex lens) or diverge (concave lens). As a result, the power of a lens can be defined as the degree of light ray convergence or divergence. The reciprocal of a lens' focal length is used to calculate its power.
 
P=1f

The dioptre is the SI unit of lens power. It is denoted by the letter D. The power of the lens is expressed in ‘D' if the focal length is expressed in ‘m'. As a result, the power of a lens with a focal length of one metre is 1D.
 
1D=1m1
 
The power of a convex lens is assumed to be positive, whereas the power of a concave lens is assumed to be negative.
 
Eyeglass-Lenses.jpg
Power of a lens