- 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 organ are found in a variety of creature. 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.
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
 Light from a single point of a distant object and light from a single point of a near object being brought to a focus. 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
 Some believe that the human eye is the window to the soul. External links
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