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Eyes are organs that detect light. The simplest "eyes", in even unicellular organisms, do nothing but detect whether the surroundings are light or dark, which is sufficient for the entrainment of circadian rhythms and may allow the organism to seek out or avoid light. More complex optical systems with resolving power have come in ten fundamentally different "designs", and 96% of animal species possess a complex optical system.[1]

"The world" is a proper noun for the planet Earth envisioned from an anthropocentric or human worldview, as a place inhabited by human beings and other terrestrial lifes. It is often used to signify the sum of human experience and history, or the human condition in general.[1] The world population is approximately 6.68 billion people.[2]

In a metaphysical context, World may refer to everything that constitutes reality and the Universe: see World (philosophy).