Eye

The eye is a round organ for sensing light so organisms can see. It is the first part of the visual system. About 97 percent of animal species have eyes.^[1] Image-resolving eyes are present in cnidaria, molluscs, vertebrates,^[2] annelids and arthropods.^[1][3]

In mammals, two kinds of cells, rods and cones, allow sight by sending signals through the optic nerve to the brain.

Some animals can see light that humans cannot see. They can see ultraviolet or infrared light.

The lens on the front part of the eye acts like a camera lens. It can be pulled flatter by muscles inside the eye, or allowed to become rounder. As some people get older, they may not be able to do this perfectly. Many people are born with other small problems or get them later in life. Eyeglasses (or contact lenses) may fix the problem.

Like different cameras, different eyes have different abilities. They may have higher or lower resolution, the ability to detect small details. They may have different performance in low light; nocturnal animals can see better at night than daytime animals. They may have different ability to distinguish colours.

1. ↑ ^{1.0 1.1} Land M.F. & Fernald R.D. 1992 (1992). "The evolution of eyes". Annual Review of Neuroscience. 15: 1–29. doi:10.1146/annurev.ne.15.030192.000245. PMID 1575438.{{cite journal}}: CS1 maint: numeric names: authors list (link)
2. ↑ Lamb, Trevor D.; Collin, Shaun P.; Pugh, Edward N. (2007). "Evolution of the vertebrate eye: Opsins, photoreceptors, retina and eye cup". Nature Reviews Neuroscience. 8 (12): 960–976. doi:10.1038/nrn2283. PMC 3143066. PMID 18026166.
3. ↑ Frentiu, Francesca D. & Briscoe Adriana D. (2008), "A butterfly eye's view of birds", BioEssays, 30 (11–12): 1151–1162, doi:10.1002/bies.20828, PMID 18937365, S2CID 34409725