| SEVERAL THOUSAND YEARS ago people believed that vision involved the emission of some kind of radiation by the eye. Everyone realized the eyes are necessary—an eye injury causes a loss of vision—but people in ancient times imagined the eye sent out rays and bounced them off distant objects, providing a sense of vision by analyzing the returning radiation. If vision really worked this way, then eyesight would result from the eye actively exploring the environment.
But vision works differently. Arab scholar Abu Ali al-Hasan Ibn Al-Haitham (965–1040) correctly proposed that the eye receives radiation emitted by other sources; some of the radiation travels straight to the eye, which makes the source visible, and some of the radiation reaches the eye after bouncing off objects that do not otherwise emit radiation, which is how these objects become visible. The major source of this radiation, called light, is the Sun. Light is the messenger of vision, and the eye is tuned to detect it. |
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