Optical Power (also known as dioptric power or refractive power) is a measure of how strongly a lens, mirror, or other optical system converges or diverges light. It is the degree to which light rays are bent when passing through a lens.

A lens with high optical power bends light rays more strongly, bringing them to a focus at a shorter distance. A lens with low optical power bends light less and has a longer focal length.

The standard unit for optical power is the Diopter (D).

Optical power is defined as the reciprocal of the focal length of the lens or mirror.

The formula is elegantly simple:

P = 1 / f

It is absolutely critical that the focal length is measured in meters (m) for this formula to give the power in diopters.

The sign of the optical power tells you the type of lens or mirror:

Positive Power (+): A positive power indicates a converging optical element. This applies to convex lenses and concave mirrors. They bring parallel light rays together.

Negative Power (-): A negative power indicates a diverging optical element. This applies to concave lenses and convex mirrors. They cause parallel light rays to spread apart.

Optical power is the standard unit used in optometry and is fundamental to the design of all optical instruments.

Eyeglasses and Contact Lenses: Your prescription is given in diopters. A negative prescription (e.g., -2.5 D) is for nearsightedness (myopia) and uses a diverging lens. A positive prescription (e.g., +2.0 D) is for farsightedness (hyperopia) and uses a converging lens.

Cameras: The power of a camera lens determines its ability to magnify an image. A telephoto lens has a high optical power (short focal length), while a wide-angle lens has a low optical power (long focal length).

Microscopes and Telescopes: These instruments work by combining multiple lenses with different optical powers to produce a highly magnified final image.