Navigation Links
Eye


This article refers to the sight organ. See Eye (disambiguation) for other usages.


Diagram of a human eye. Note that not all eyes have the same anatomy as a human eye.

An eye is an organ that detects light. Different kinds of light-sensitive organs are found in a variety of creatures. The simplest eyes do nothing but detect whether the surroundings are light or dark. More complex eyes are used to provide the sense of vision.

Contents

Varieties of eye

Compound eyes are found among the arthropods (insects and kin), and are composed of many simple facets which give a pixelated image (not multiple images as is often believed). Trilobites (now extinct) had a unique form of eye (usually compound) formed from crystals of calcite, incorporating a doublet structure that gave a good field of view despite the rigid lens.

In most vertebrates and some mollusks (such as octopuses) the eye works by projecting images onto a light-sensitive retina, where the light is detected and transmitted to the brain via the optic nerve. The eye is typically roughly spherical, filled with a transparent gel-like substance called the vitreous humour, with a focusing lens and often a muscle called the iris that controls how much light enters. Although they are quite similar in function and appearance once fully developed, vertebrate eyes grow outward from brain cells during embryonic development, while mollusk eyes grow inward from skin cells.

How a complex structure like the eye could have evolved is often said to be a difficult question for the theory of evolution, on the basis that intermediate forms of an eye would presumably have been of little use, and light-sensitive organs are present in a variety of different creatures without any clear evolutionary link. However, eyes in different levels various animals show adaption to their requirements (for example, birds of prey have much greater visual acuity than humans), and the different forms of eye in, for example, vertebrates and mollusks are often cited as examples of parallel evolution.

Focusing


In order for light rays to be brought to a focus they must be refracted. The amount of refraction required depends on the distance of the object which is being viewed. A distant object will require less bending of light than a nearer one. Most of the refraction occurs at the cornea which has a fixed curvature. The remainder of the required refraction occurs at the lens. The lens can be pulled flatter or rounder by muscles, which adjust the power of the lens. As we age we lose this ability to adjust the focus. Such a condition is known as presbyopia. There are other refraction errors arising from the shape of the cornea and lens, and from the length of the eyeball. These include myopia, hyperopia, and astigmatism.

Convergence


When a person stares at an object, the two eyeballs rotate sideways to point to the object, so that the object appears at the center of the image formed in each eye's retina. In order to look at a nearby object, the two eyeballs rotate towards each other so that their eyesight can converge on the object. This is generally referred to as cross-eyed viewing. To see a faraway object, the two eyeballs diverge to become almost parallel to each other. This is known as wall-eyed viewing, where the convergence angle is much smaller than that in a cross-eyed viewing.

Normally, focusing operations of the eyes are coupled to the convergence operations. That is, when looking at a nearby object, the brain automatically increases the refraction index of lenses to focus on the object. At the same time, the brain also rotates the two eyeballs towards each other so they converge on the object.

Ocular Anatomy

Eye Related Problems

See also

The human eye is sometimes metaphorically called "the window to the soul."

External links


'"/>


(Date:9/2/2015)... , September 2, 2015 ... purpose implied in a system which uses identical ... signature, iris, vein, DNA and finger print. Increasing ... to develop such system which provides high level ... PIN, Patterns and tokens are vulnerable to attacks. ...
(Date:8/31/2015)... 2015 Growing need for data ... digitization projects to drive India biometrics ... Research report, " India Biometrics Market Forecast & Opportunities, 2020 ... projected to grow at a CAGR of over 35% during ... account of extensive use of biometric technology in the government ...
(Date:8/28/2015)... PUNE, India , August 28, 2015 ... "Cognitive Assessment and Training Market by Assessment Type (Pen ... Classroom Learning, Brain Training, Corporate Learning, Academic Research), Vertical ... MarketsandMarkets, Cognitive Assessment and Training Market to grow from ... by 2020, at a Compound Annual Growth Rate (CAGR) ...
Breaking Biology News(10 mins):Biometrics Technology Market is Expected to Reach $13.8 Billion by 2015: Hexa Research Inc. 2Biometrics Technology Market is Expected to Reach $13.8 Billion by 2015: Hexa Research Inc. 3Biometrics Technology Market is Expected to Reach $13.8 Billion by 2015: Hexa Research Inc. 4India Biometrics Market to Grow at Over 35% Through 2020, Says TechSci Research 2India Biometrics Market to Grow at Over 35% Through 2020, Says TechSci Research 3Cognitive Assessment and Training Market Worth 7.5 Billion USD by 2020 2Cognitive Assessment and Training Market Worth 7.5 Billion USD by 2020 3Cognitive Assessment and Training Market Worth 7.5 Billion USD by 2020 4
... The Aeras Global TB Vaccine Foundation is pleased to ... Food and Drug Administration (FDA) to develop new biological ... grant, which supports the coordination of a multi-investigator project ... and South Africa, will provide $362,102 in the first ...
... 4 October, 2010 Elsevier Properties SA today ... Alexander (Sandy) Lawson, for receiving the 2011 Herman ... (ACS) Division of Chemical Information (CINF), the award ... theory and practice of chemical information science and ...
... joint work and scientific adventure, marine explorers from more ... Biological Laboratory (MBL), today delivered the first global Census ... lives and hides in the world,s oceans. In one ... 2,700 scientists spent over 9,000 days at sea on ...
Cached Biology News:Aeras-led research consortium receives FDA support 2Professor Dr. Alexander (Sandy) Lawson is awarded the 2011 Herman Skolnik Award 2MBL scientists reveal findings of World Ocean Microbe Census 2MBL scientists reveal findings of World Ocean Microbe Census 3
Other biology definitionOther Tags