1 dioptre of power of a lens can be defines as :

You must understand that the lens's power is inversely proportional to its focal length in order to solve this problem. The SI unit of power on the lens is the dioptre, which is indicated by the letter D in this context. You may have noticed that a convex lens has a positive power while a concave lens has a negative power.

You should be aware that a dioptre is a unit of refractive power and that it is equivalent to the lens's corresponding focal length. The lens's ability to focus the light that strikes it determines how strong it is. Power $P=\frac{1}{\text{ focal length }\left(\mathrm{f}\right)}=1\mathrm{D}$

A lens's power determines how well it can focus light rays that strike it.
The power of a lens with a 1 m focal length is one dioptre.