{{short description|Degree to which an optical system converges or diverges light}} {{About||magnifying power|Magnification|the power of light|Radiant flux|and|Luminous flux}} {{Use British English|date=December 2024}} [[File:optical_power_of_a_lens.svg|thumb|Illustration of the relationship between optical power in [[dioptre]]s and [[focal length]] in metres]]
In [[optics]], '''optical power''' (also referred to as '''dioptric power''', '''refractive power''', '''focal power''', '''focusing power''', or '''convergence power''') is the degree to which a [[lens (optics)|lens]], [[mirror]], or other optical system converges or diverges light.<ref>{{Cite book |last=Basu |first=Dipak |url=https://books.google.com/books?id=-QhAkBSk7IUC&pg=PA210 |title=Dictionary of Pure and Applied Physics |date=2018-10-08 |publisher=CRC Press |isbn=978-1-4200-5022-6 |language=en}}</ref> It is equal to the [[Multiplicative inverse|reciprocal]] of the [[focal length]] of the device; high optical power corresponds to short focal length. The SI unit for optical power is the [[inverse metre]] ({{val|u=m-1}}), which is also called a ''[[dioptre]]'' (symbol: dpt or D) when used as a unit of optical power.
==Explanation== The optical power {{mvar|P}} of a device is related to its focal length {{mvar|f}} by {{math|1=''P'' = 1/''f''}}.<ref>{{cite book |first= John E. |last= Greivenkamp |year= 2004 |title= Field Guide to Geometrical Optics |publisher= SPIE |series= SPIE Field Guides |volume= FG01 |isbn= 0-8194-5294-7 |page= 7 }}</ref>
[[Converging lens]]es have positive optical power, while [[diverging lens]]es have negative power. When a lens is immersed in a [[medium (optics)|refractive medium]], its optical power and focal length change.<!--A description of how they change, with reference to a reliable source, would be useful here.-->
For two or more [[thin lens]]es close together, the optical power of the combined lenses is approximately equal to the sum of the optical powers of each lens: {{math|1=''P'' = ''P''<sub>1</sub> + ''P''<sub>2</sub>}}. 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]].
An [[eye]] that has too much or too little refractive power to [[focus (optics)|focus]] light onto the [[retina]] has a [[refractive error]]. 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 (vision)|meridian]] that is different from the refractive power of the other meridians has [[Astigmatism (eye)|astigmatism]]. This is also known as a cylindrical power. [[Anisometropia]] is the condition in which one eye has a different refractive power than the other eye.
==See also== *[[Accommodation of the eye]] *[[Lens clock]] *[[Lensmeter]] *[[Plate scale]] *[[Vergence (optics)|Vergence]] *[[Vertometer]]
==References== {{Reflist}}
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[[Category:Optical quantities|Power]]