Optical power (also referred to as dioptric power, refractive 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 of the device. The dioptre is the most common unit of measurement of optical power. The SI unit for optical power is the inverse metre (m−1).
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.
When a lens is immersed in a refractive medium, its optical power and focal length change.
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. 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. Anisometropia is the condition in which one eye has a different refractive power than the other eye.
Famous quotes containing the words optical and/or power:
“It is said that a carpenter building a summer hotel here ... declared that one very clear day he picked out a ship coming into Portland Harbor and could distinctly see that its cargo was West Indian rum. A county historian avers that it was probably an optical delusion, the result of looking so often through a glass in common use in those days.”
—For the State of New Hampshire, U.S. public relief program (1935-1943)
“His nature is too noble for the world;
He would not flatter Neptune for his trident,
Or Jove for’s power to thunder.”
—William Shakespeare (1564–1616)