Eye
From Wikipedia, the free encyclopedia
Eyes are organs that detect light and convert it into electro-chemical impulses in neurons. The simplest photoreceptor cells in conscious vision connect light to movement. In higher organisms the eye is a complex opticalsystem which collects light from the surrounding environment, regulates its intensity through a diaphragm, focuses it through an adjustable assembly of lenses to form an image, converts this image into a set of electrical signals, and transmits these signals to the brain through complex neural pathways that connect the eye via the optic nerve to the visual cortex and other areas of the brain. Eyes with resolving power have come in ten fundamentally different forms, and 96% of animal species possess a complex optical system.[1] Image-resolving eyes are present in molluscs, chordates and arthropods.[2]
The simplest "eyes", such as those in microorganisms, do nothing but detect whether the surroundings are light or dark, which is sufficient for the entrainment of circadian rhythms.[citation needed] From more complex eyes, retinal photosensitive ganglion cells send signals along the retinohypothalamic tract to the suprachiasmatic nuclei to effect circadian adjustment.
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Compound eyes
A compound eye may consist of thousands of individual photoreceptor units or ommatidia (ommatidium, singular). The image perceived is a combination of inputs from the numerous ommatidia (individual "eye units"), which are located on a convex surface, thus pointing in slightly different directions. Compared with simple eyes, compound eyes possess a very large view angle, and can detect fast movement and, in some cases, the polarisation of light.[24](Even the trained human eye can determine the orientation of polarized light which manifests in a phenomenon called Haidinger's brush.) Because the individual lenses are so small, the effects of diffraction impose a limit on the possible resolution that can be obtained (assuming that they do not function as phased arrays). This can only be countered by increasing lens size and number. To see with a resolution comparable to our simple eyes, humans would require ridiculously large compound eyes, around 11 m in radius.[25]
Compound eyes fall into two groups: apposition eyes, which form multiple inverted images, and superposition eyes, which form a single erect image.[26] Compound eyes are common in arthropods, and are also present in annelids and some bivalved molluscs.[27] Compound eyes, in arthropods at least, grow at their margins by the addition of new ommatidia.[28]
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