Eye

Eyes are organs that detect light, and send signals along the optic nerve to the visual and other areas of the brain. Complex optical systems with resolving power have come in ten fundamentally different forms, and 96% of animal species possess a complex optical system.

The simplest "eyes", such as those in unicellular organism, do nothing but detect whether the surroundings are light or dark, which is sufficient for the entrainment of circadian rhythms.