Define 1 D of power of a lens.

1 D is the power of a convex lens of 1m focal length.

Power of a lens is the ability of the lens to converge or diverge ray of light.

Power of lens, P=

Where,

F is the focal length of the lens

On solving for F, we get

Thus, the 1 D is the power of a covex lens of focal length 1m