Everything about Emission Theory Vision totally explained
Emission theory or
extramission theory 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 This theory has been replaced by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 is that visual perception is accomplished by rays of light reflected from objects into 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.
History
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.
(External Link
).
In about 300 BCE,
Euclid wrote
Optica, in which he studied the properties of light. Euclid postulated that light travelled in straight lines and he 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. Of course if the beam from the eye travels infinitely fast this isn't a problem.
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're 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 and 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.
Evidence for the theory
Adherents of emission theory cited at least two lines of evidence for it.
The custom of
saluting is said by some to stem from the habit of Greek soldiers putting their hands up in front of their eyes to "shade" their eyes from the powerful "light" shining from the eyes of their commanders. The light from the eyes of some animals (such as cats, which modern science has determined merely have highly reflective eyes) could also be seen in "darkness". Adherents of intromission theory countered by saying that if emission theory were true, then someone with weak eyes should have his or her vision improved when someone with good eyes looks at the same objects.
Most argue that Euclid's version of emission theory was purely metaphorical, highlighting only the geometrical relations between eyes and objects. The geometry of classical
optics is equivalent no matter which direction light is considered to be moving in, since light is modeled by its path, not as a moving object. (Direction of propagation is important, however, in the modern theory of
special relativity.)
Measuring the
speed of light was one line of evidence that spelt the end of emission theory as anything other than a metaphor.
Persistence of the theory
Winer et al. (2002) have found recent evidence that as many as 50% of American college students believe in emission theory.
Modern
ray-tracing computer programs often trace lines of sight from "eyes" to "objects" and thence to light sources to determine the
colour and
luminance of
pixels in a simulated scene. This avoids the extra computation that would be required to trace rays which don't intersect with "eyes", and which by definition can't be "seen".
Further Information
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