The eye is an organ of the visual system. They provide vision for organisms, the ability to process visual details, and the realization of a variety of light response functions independent of vision. The eye detects light and converts it into an electrochemical pulse in a neuron. In higher biology, the eye is a complex optical system, which collects light from the surrounding environment, adjusts its intensity through aperture, focuses it through adjustable lens components to form an image, then converts the image into a set of electrical signals, and transmits these signals to the brain through complex neural pathways that connect the eye through the optic nerve To the visual cortex and other areas of the brain. There are ten fundamentally different forms of eye with resolution, and 96% of animal species have complex optical systems. There are image discrimination eyes in molluscs, chordates and arthropods.

The simplest "eyes," such as those in microbes, do nothing but detect whether the surrounding environment is bright or dark, enough to induce circadian rhythms. The photoreceptor ganglion cells of retina send signals to the suprachiasmatic nucleus from the more complex eyes along the hypothalamic tract of retina to regulate the circadian rhythm and reach the anterior covering area to control the light reflection of pupil.