Eye
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- This article refers to the sight organ. See Eye (disambiguation) for other usages.
An eye is an organ which has evolved for the purpose of detecting light. 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.
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).
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Diagram of a human eye. Note that not all eyes have the same anatomy as a human eye.
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Light from a single point of a distant object and light from a single point of a near object being brought to a focus
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The human eye is said to be the window to the soul
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In most vertebrates and some mollusks 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.
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.
Parts of the eye
Problems
See also
