Emission theory (vision)

Emission theory or extramission theory (variants: extromission, extromittism) is the proposal that visual perception is accomplished by rays of light emitted by the eyes. This theory has been replaced by intromission theory, which states that visual perception comes from something representative of the object (later established to be rays of light reflected from it) entering the eyes. Modern physics has confirmed that light is physically transmitted by photons from a light source, such as the sun, to visible objects, and finishing with the detector, such as a human eye or camera.

In the fifth century BCE, Empedocles postulated that everything was composed of four elements; fire, air, earth, and water. He believed that Aphrodite made the human eye out of the four elements and that she lit the fire in the eye which shone out from the eye, making sight possible. If this were true, then one could see during the night just as well as during the day, so Empedocles postulated an interaction between rays from the eyes and rays from a source such as the sun.

Around 400 BCE, emission theory was held by Plato.

Around 300 BCE, Euclid wrote Optica, in which he studied the properties of light. Euclid postulated that light travelled in straight lines and described the laws of reflection and studied them mathematically. He questioned that sight is the result of a beam from the eye, for he asked how one sees the stars immediately, if one closes one's eyes, then opens them at night.

In 55 BCE, Lucretius, a Roman who carried on the ideas of earlier Greek atomists, wrote:

The light and heat of the sun; these are composed of minute atoms which, when they are shoved off, lose no time in shooting right across the interspace of air in the direction imparted by the shove. – On the nature of the Universe

Despite being similar to later particle theories, Lucretius's views were not generally accepted; light was still theorized as emanating from the eye.

Ptolemy (c. 2nd century) wrote about the refraction of light and developed a theory of vision that objects are seen by rays of light emanating from the eyes.

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