Define 1 dioptre of power of a lens.

The dioptre (or diopter) is a unit of measurement used to express the optical power of a lens. The optical power of a lens is a measure of its ability to converge or diverge light. It is defined as the reciprocal of the focal length of the lens in meters.

The formula for calculating the optical power (P) of a lens in dioptres is:

P = 1/f

where:

» P is the optical power of the lens in dioptres.
» f is the focal length of the lens in meters.
So, 1 dioptre of lens power corresponds to a lens with a focal length of 1 meter. Lenses with positive power (converging lenses) bring light rays together, while lenses with negative power (diverging lenses) cause light rays to spread out.

For example:

» A converging lens with a focal length of 0.5 meters has an optical power of P = 1/0.5 = 2 dioptres.
A diverging lens with a focal length of -1 meter also has an optical power of P = 1/−1 =−1 dioptre.

Optometrists use dioptres to prescribe corrective lenses for individuals with vision problems. The prescription for glasses or contact lenses is given in terms of the required optical power to correct refractive errors such as nearsightedness (myopia), farsightedness (hyperopia), or astigmatism.