Eye (organ of vision)

Structure and function

Typical image-forming eyes share several parts: a transparent front surface (cornea) that begins to focus incoming light; a variable aperture (pupil) controlled by the pigmented iris; a lens that fine-tunes focus; and an inner lining (retina) containing photoreceptor cells. In vertebrates the two main photoreceptors are rods, which are highly sensitive and support vision in dim light, and cones, which mediate colour and fine detail under brighter conditions. Photoreceptor signals are transmitted along retinal neurons to form the optic nerve, which carries information to visual centers in the brain. Muscles that change the shape of the lens permit accommodation — adjusting focus for near or far objects. Additional structures such as tear-producing glands, eyelids, and chambers filled with fluid help maintain optical clarity and ocular health.

Diversity and evolution

Eyes have evolved independently many times and show striking diversity. Arthropods like insects and crustaceans often have compound eyes made of many repeating units (ommatidia) that excel at detecting motion and providing a wide field of view. Molluscs and vertebrates possess single-chamber image-forming eyes with a camera-like arrangement. Some simple animals have non-image-forming photoreceptors that guide daily rhythms and light-avoidance behaviors. Evolutionary convergence — different lineages evolving similar eye solutions — illustrates how powerful selective pressures have shaped visual systems.

Sensitivities beyond human vision

Not all vision matches human perception. Many species detect wavelengths outside the human visible band: some birds, insects and fish can see ultraviolet light, which aids foraging and signalling, while certain snakes sense infrared energy to locate warm prey. Visual systems also differ in temporal resolution (how quickly they detect changes), spectral sensitivity (which colours are distinguishable), and capacity for low-light vision; nocturnal animals typically have retinas and optics adapted to gather more light at the expense of colour discrimination.

Human vision and common conditions

Human eyes provide high-resolution, colour vision under a range of lighting conditions, but they are susceptible to optical and neurological disorders. Refractive errors such as myopia, hyperopia and astigmatism arise when the eye does not focus light precisely on the retina; presbyopia is an age-related loss of accommodation. Other common issues include cataracts (clouding of the lens), glaucoma (pressure-related nerve damage), and retinal disorders. Many conditions are managed with corrective lenses or surgical procedures, and public-health measures and clinical screening help preserve vision at the population level.

Importance, research and further reading

Vision shapes behaviour, ecology and culture: it influences navigation, communication, predation and mate choice. Scientific study of the eye spans anatomy, physiology, genetics and optics, and it continues to inform technologies such as cameras, sensors and medical treatments. For concise introductions and more detailed resources, consult the links below.

• Light detection and basic eye function
• The visual system and neural pathways
• Distribution of eyes across animals
• Examples from cnidarians
• Eyes in molluscs
• Vertebrate eye anatomy
• Visual systems in annelids
• Arthropod compound eyes
• Mammalian retinal structure
• The optic nerve and brain connections
• Ultraviolet vision in animals
• Infrared sensing and thermoreception
• Optical principles and the eye as a lens system
• Accommodation and lens mechanics
• Corrective lenses and common treatments