Optical power

For the power of light, see Radiant flux and Luminous flux.

Optical power (dioptric power or refractive power) is the degree to which a lens or mirror 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 well approximated by the sum of the individual 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 ophthalmology.

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.

Its a misconcept that Power of a lens is actually equal to inverse of focal length. Infact power of a lens depends on the refractive index of the medium.

Power in a medium = (refrative index of medium) / (Focal length of lens in that medium).

The focal length of the lens in medium is found by lens makers formula, and the refractive index of medium should be known. The misconcept emerged because of the fact that refractive index of air (or vacuum) is one.

See also

• Vergence
• Lens clock
• Vertometer
• Lensmeter