FACTOID # 165: Bolivia has 4,500 Navy personnel - which seems like quite a lot for a landlocked country.
 
 Home   Encyclopedia   Statistics   Countries A-Z   Flags   Maps   Education   Forum   FAQ   About 
 
 
 
WHAT'S NEW
RECENT ARTICLES
More Recent Articles »
 

SEARCH ALL

FACTS & STATISTICS    Advanced view

Search encyclopedia, statistics and forums:

 

 

(* = Graphable)

 

 


Encyclopedia > Accommodation (eye)
Light from a single point of a distant object and light from a single point of a near object being brought to a focus by changing the curvature of the lens.
Light from a single point of a distant object and light from a single point of a near object being brought to a focus by changing the curvature of the lens.

Accommodation is the process by which the eye increases optical power to maintain a clear image (focus) on the retina.[1] The principal focusing ability of the (terrestrial) eye is due to the difference in refractive index between air and the curved cornea, but the variable curvature of the lens allows for an additional adjustment. This varies from a maximum of over 15 diopters in an infant to only about 1.5 diopters in a person 70 years old, as the lens becomes less flexible with age.[citation needed] Image File history File links Focus_in_an_eye. ... Image File history File links Focus_in_an_eye. ... A human eye. ... Optical power or dioptric power or refractive power is the degree to which a lens or mirror converges or diverges light. ... An image that is partially in focus, but mostly out of focus in varying degrees. ... Human eye cross-sectional view. ... The refractive index (or index of refraction) of a material is the factor by which the phase velocity of electromagnetic radiation is slowed in that material, relative to its velocity in a vacuum. ... Layers of Atmosphere (NOAA) Air redirects here. ... The cornea is the transparent front part of the eye that covers the iris, pupil, and anterior chamber, providing most of an eyes optical power [1]. Together with the lens, the cornea refracts light and, as a result, helps the eye to focus. ... Light from a single point of a distant object and light from a single point of a near object being brought to a focus by changing the curvature of the lens. ... A dioptre, or diopter, is a non-SI unit of measurement of the optical power of a lens or curved mirror, which is equal to the reciprocal of the focal length measured in metres (i. ... A human infant An infant or baby is a person younger than a toddler. ... For other uses, see Flexibility (disambiguation). ... It has been suggested that Longevity genes be merged into this article or section. ...

Contents

Theories of mechanism

  • Helmholtz - The most widely held theory of accommodation is that proposed by Hermann von Helmholtz in the middle of the 19th century. When viewing a far object, the circularly arranged ciliary muscle relaxes causing the lens zonules and suspensory ligaments to pull on the lens, flattening it. The source of the tension is the pressure that the vitreous and aqueous humours exert outwards onto the sclera. When viewing a near object, the ciliary muscles contract (resisting the outward pressure on the sclera) causing the lens zonules to slacken which allows the lens to spring back into a thicker, more convex, form.
  • Schachar - Ronald Schachar argues that, in opposition to Helmholtz's theory, the ciliary muscle actually pulls on the crystalline lens' equator in an outwardly radial manner, and the consequent changes in lens shape increase the lens power.[3] Counterintuitive indeed, this pulling reportedly steepens central curvature and flattens peripheral curvature. He goes further to explain that it is not sclerosing or stiffening of the crystalline lens that linearly reduces accommodative amplitude, rather that age-related increase in lens diameter reduces the space between ciliary body and lens, linearly reducing the effectivity of the ciliary body's effect on lens dynamics. The "Scleral Expansion Band" technique for the surgical reversal of presbyopia is based on this theory, however, the efficacy of the procedure has been met with skepticism.[4]

Hermann Ludwig Ferdinand von Helmholtz (August 31, 1821 – September 8, 1894) was a German physician and physicist. ... Grays FIG. 872- The choroid and iris. ... Light from a single point of a distant object and light from a single point of a near object being brought to a focus by changing the curvature of the lens. ... The zonule of Zinn is a ring of fibrous strands connecting the ciliary body with the crystalline lens of the eye. ... Schematic diagram of the human eye. ... Year 1920 (MCMXX) was a leap year starting on Thursday. ... William Horatio Bates (December 23, 1860 - July 10, 1931) was an American physician who developed what is now known as the Bates Method of eye exercises. ... The extraocular muscles are the six muscles that control the movements of the eye. ... The superior oblique muscle is a muscle in the orbit that causes the eye to look downwards when it is already directed medially (looking towards the nose). ... The inferior oblique muscle is a muscle in the orbit that adducts (medially rotates) and elevates the eyeball. ... Presbyopia is the eyes diminished power of accommodation that occurs with aging. ...

Accommodative dysfunction

Duke-Elder classified a number of accommodative dysfunctions:[5]

Accommodative insufficiency (AI) involves the inability of the eye to focus properly on an object. ... This article or section does not adequately cite its references or sources. ...

References

  1. ^ Cassin, B. and Solomon, S. Dictionary of Eye Terminology. Gainsville, Florida: Triad Publishing Company, 1990.
  2. ^ http://www.iblindness.org/books/bates/ch4.html
  3. ^ Burd HJ, Judge SJ, Flavell MJ. "Mechanics of accommodation of the human eye." Vision Res. 1999 May;39(9):1591-5. PMID 10343853.
  4. ^ http://aglasser.opt.uh.edu/presbyopia/presbyopia.html
  5. ^ Duke-Elder, Sir Stewart (1969). The Practice of Refraction (8th ed.). St. Louis: The C.V. Mosby Company. ISBN 0-7000-1410-1.
  • Indiana University School of Optometry

See also

Disorders of accommodation

Person exhibiting esotropia of the right eye Esotropia is a form of strabismus where one or both of the eyes turn inward. ... This article or section does not adequately cite its references or sources. ... Hyperopia, also known as hypermetropia or colloquially as farsightedness or longsightedness, is a defect of vision caused by an imperfection in the eye (often when the eyeball is too short or when the lens cannot become round enough), causing inability to focus on near objects, and in extreme cases causing... Presbyopia is the eyes diminished power of accommodation that occurs with aging. ... Pseudomyopia refers to an intermittent and temporary shift in refraction of the eye towards myopia. ...

Other

Human eye The Accommodation Reflex is a reflex action of the eye, in response to focusing on a near object, then looking at distant object (and vice versa). ... Amplitude of accommodation (AA) is a measurement of the eye’s ability to focus clearly on objects at near distances (i. ... Cycloplegia is the paralysis of the ciliary muscle, resulting in a loss of accommodation. ... Cyclospasm is the contraction of the ciliary muscle in the eye, in the accommodation of focus for near vision. ... The Edinger-Westphal nucleus is the accessory parasympathetic nucleus of the oculomotor nerve, supplying the constricting muscles of the iris. ... Negative relative accommodation (NRA) is a measure of the maximum ability to relax accommodation while maintaining clear, single binocular vision [1]. See also Amplitude of accommodation Convergence insufficiency Eye examination Positive relative accommodation Category: ... Positive relative accommodation (PRA) is a measure of the maximum ability to stimulate accommodation while maintaining clear, single binocular vision [1]. See also Amplitude of accommodation Eye examination Convergence insufficiency Negative relative accommodation Category: ...

External links


  Results from FactBites:
 
Glossary (5619 words)
Accommodation- (eye focusing) the eye's ability to adjust its focus by the action of the ciliary muscle, which increases the lens focusing power.
The ability of the eye to accommodate does decrease with age due to the crystalline lens becoming less flexible causing a condition called presbyopia.
The eyes are filled with fluid which maintains the shape of the eye, supplies it with nutrition and aids in the focusing of light.
Eye - Psychology Wiki - a Wikia wiki (4709 words)
In most vertebrates and some mollusks, the eye works by allowing light to enter it and project onto a light-sensitive panel of cells known as the retina at the rear of the eye, where the light is detected and converted into electrical signals, which are then transmitted to the brain via the optic nerve.
Such eyes are typically roughly spherical, filled with a transparent gel-like substance called the vitreous humour, with a focusing lens and often an iris which regulates the intensity of the light that enters the eye.
The common origin (monophyly) of all animal eyes is now widely accepted as fact based on shared anatomical and genetic features of all eyes; that is, all modern eyes, varied as they are, have their origins in a proto-eye believed to have evolved some 540 million years ago.
  More results at FactBites »


 
 

COMMENTARY     


Share your thoughts, questions and commentary here
Your name
Your comments

Want to know more?
Search encyclopedia, statistics and forums:
 


Lesson Plans | Student Area | Student FAQ | Reviews | Press Releases |  Feeds | Contact
The Wikipedia article included on this page is licensed under the GFDL.
Images may be subject to relevant owners' copyright.
All other elements are (c) copyright NationMaster.com 2003-5. All Rights Reserved.
Usage implies agreement with terms, 1022, m