optics Optics is the branch of physics that studies the behaviour and properties of light, including its interactions with matter and the construction of optical instruments, instruments that use or Photodetector, detect it. Optics usually describes t ...

, optical power (also referred to as dioptric power, refractive power, focal power, focusing power, or convergence power) is the degree to which a

lens A lens is a transmissive optical device that focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a compound lens consists of several simple lenses (''elements'') ...

mirror A mirror, also known as a looking glass, is an object that Reflection (physics), reflects an image. Light that bounces off a mirror forms an image of whatever is in front of it, which is then focused through the lens of the eye or a camera ...

, or other optical system converges or diverges light. It is equal to the reciprocal of the

focal length The focal length of an Optics, optical system is a measure of how strongly the system converges or diverges light; it is the Multiplicative inverse, inverse of the system's optical power. A positive focal length indicates that a system Converge ...

of the device; high optical power corresponds to short focal length. The SI unit for optical power is the inverse metre (), which is also called a ''

dioptre A dioptre ( British spelling) or (American spelling), symbol dpt or D, is a unit of measurement with dimension of reciprocal length, equivalent to one reciprocal metre, . It is normally used to express the optical power of a lens or curved mi ...

'' (symbol: dpt or D) when used as a unit of optical power.

Explanation

The optical power of a device is related to its focal length by .

Converging lens A lens is a transmissive optical device that focuses or disperses a light beam by means of refraction. A simple lens consists of a single piece of transparent material, while a compound lens consists of several simple lenses (''elements''), ...

es have positive optical power, while

diverging lens Divergence is a mathematical function that associates a scalar with every point of a vector field. Divergence, divergent, or variants of the word, may also refer to: Mathematics * Divergence (computer science), a computation which does not ter ...

es have negative power. When a lens is immersed in a refractive medium, its optical power and focal length change. For two or more

thin lens In optics, a thin lens is a lens (optics), lens with a thickness (distance along the optical axis between the two surfaces of the lens) that is negligible compared to the radius of curvature (optics), radii of curvature of the lens surfaces. Len ...

es close together, the optical power of the combined lenses is approximately equal to the sum of the optical powers of each lens: . Similarly, the optical power of a single lens is roughly equal to the sum of the powers of each surface. These approximations are commonly used in

optometry Optometry is the healthcare practice concerned with examining the eyes for visual defects, prescribing corrective lenses, and detecting eye abnormalities. In the United States and Canada, optometrists are those that hold a post-baccalaureate f ...

. An

eye An eye is a sensory organ that allows an organism to perceive visual information. It detects light and converts it into electro-chemical impulses in neurons (neurones). It is part of an organism's visual system. In higher organisms, the ey ...

that has too much or too little refractive power to

focus Focus (: foci or focuses) may refer to: Arts * Focus or Focus Festival, former name of the Adelaide Fringe arts festival in East Australia Film *Focus (2001 film), ''Focus'' (2001 film), a 2001 film based on the Arthur Miller novel *Focus (2015 ...

light onto the

retina The retina (; or retinas) is the innermost, photosensitivity, light-sensitive layer of tissue (biology), tissue of the eye of most vertebrates and some Mollusca, molluscs. The optics of the eye create a focus (optics), focused two-dimensional ...

has a

refractive error Refractive error is a problem with focus (optics), focusing light accurately on the retina due to the shape of the eye and/or cornea. The most common types of refractive error are myopia, near-sightedness, hyperopia, far-sightedness, astigmatis ...

. A myopic eye has too much power so light is focused in front of the retina. This is noted as a minus power. Conversely, a hyperopic eye has too little power so when the eye is relaxed, light is focused behind the retina. An eye with a refractive power in one meridian that is different from the refractive power of the other meridians has

astigmatism Astigmatism is a type of refractive error due to rotational asymmetry in the eye's refractive power. The lens and cornea of an eye without astigmatism are nearly spherical, with only a single radius of curvature, and any refractive errors ...

. This is also known as a cylindrical power.

Anisometropia Anisometropia is a condition in which a person's eyes have substantially differing refractive power. Generally, a difference in power of one diopter (1D) is the threshold for diagnosis of the condition. Patients may have up to 3 diopters of anis ...

is the condition in which one eye has a different refractive power than the other eye.

References

{{Authority control Power

Explanation

See also

References