In
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,
mirror, 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'' (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 .
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 lenses 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 that has too much or too little refractive power to
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 that is different from the refractive power of the other meridians has
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
*
Vertometer
References
{{Authority control
Power