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Ancient Greeks and ancient Indians believed that the way in which vision works is that a beam goes out of the eye and hits an object. Whereas now we know that light reflects off the objects and then gets absorbed by the eye. My question is, how was the Ancient Greek theory of vision disproved? That is, what experiments showed that the light goes from the object to the eye and not from the eye to the object?

Wikipedia just says this:

Measuring the speed of light was one line of evidence that spelled the end of emission theory as anything other than a metaphor.

But I don't think the finiteness of the speed of light is relevant at all. The ancient Greeks already believed that light leaves the Sun before it enters the eye, and that light leaves a lamp before it enters the eye. But they thought there were two kinds of light, lux and lumen. One is the kind of light emitted by the Sun, lamps, fire, etc. and the other is the light emitted by the eye which hits objects. (I forgot which is which).

In any case, what experiments show that the light is present at an object before it's absorbed by the eye?

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  • $\begingroup$ Well, to start with, if there's line of sight between a person and a light source, but not between that light source and a object, the person cannot see the object. That is, shining a flashlight in your eyes won't let you see in the dark. I assume the ancient civilizations had an explanation for this, though? $\endgroup$ – Obie 2.0 May 25 '18 at 16:04
  • $\begingroup$ @Obie2.0 Yes, they did have an explanation for this. They thought eye beams could only reach objects illuminated by external light. $\endgroup$ – Keshav Srinivasan May 25 '18 at 17:15
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    $\begingroup$ Would History of Science and Mathematics be a better home for this question? We can migrate it for you if you'd like. $\endgroup$ – rob Jun 26 '18 at 16:27
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    $\begingroup$ What was the explanation for not being able to see anything when it was dark (ie. Deep in a cave)? It seems that the eye-particle-emitting theory, although simple in principle, needs alot of work-arounds and can be excluded with Occam's razor (when two theories can explain the same phenomena but one is more complicated, you should go for the simpler explanation). $\endgroup$ – Gyromagnetic Jun 29 '18 at 6:37
  • $\begingroup$ @rob Yes, can you move it to History of Science and Mathematics for me? $\endgroup$ – Keshav Srinivasan Jul 2 '18 at 18:35
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It was disproved by studying the anatomy of the human eye, specifically by understanding painful effects of light on the eye, such as looking at a bright object and noticing that it leaves an after-image "burned" into the eye that one can still see when looking at a darker place.

The development of a pure intromission theory (that light only comes from objects to the eye) was slow, passing through a phase of a mixed extra-/intro-mission theory. (See Edward Grant, A Source Book in Medieval Science pp. 392ff.)

Roger Bacon (1217-1292) was a pivotal figure who made

an important contribution to the history of physiological optics in the West by his exposition of Ibn al-Hyatham’s account of the eye as an image-forming device, basing his ocular anatomy on Ḥunayn ibn Isḥaq and Ibn Sīnā. … Bacon provided a functional qualitative geometry of the eye and vision. He was committed to an intromission theory of vision but he combined it with an extramission theory of vision that avoided the anthropomorphisms of earlier theories. He used the extramission theory mainly to emphasize the active role of the eye in vision.

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  • $\begingroup$ Was the camera obscura involved in this process? It gives a fairly clear demonstration that it cannot be an eye-emitted ray. $\endgroup$ – Anders Sandberg May 25 '18 at 17:01
  • $\begingroup$ Why would the fact that bright light causes pain to the eye and creates an afterimage disprove the extramission theory? $\endgroup$ – Keshav Srinivasan Jun 2 '18 at 0:47
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    $\begingroup$ @Keshav Srinivasan : How about increasing the reflection? E.g. mirrors. That's an object which light hits, as opposed to being a light source. You catch the Sun in a mirror, and it will hurt like you were looking at it directly. And if a mirror seems too "special", how about coming indoors after being out on a sunny, snowy winter day? Effectively that's like an "afterimage" across your entire visual field (and if you close your eyes, you will notice a similar faint greenish hue.). (cont'd) $\endgroup$ – The_Sympathizer Jun 29 '18 at 5:06
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    $\begingroup$ (cont'd) Eventually you have to keep adding so many exceptions that you end up eliminating everything as an "ordinary" object which the eye sends light to and the simpler theory - that the light is going from object to eye always - wins out. $\endgroup$ – The_Sympathizer Jun 29 '18 at 5:06
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    $\begingroup$ And another: what happens if you put two sets of eyes together, looking at each other? If there are special visual beams coming out of people's eyes in general, then why don't eyes look like headlights? I suppose you could then say that each person's eyes emit a unique kind of beam keyed to that person alone, but then again, we're adding more and more exceptions. $\endgroup$ – The_Sympathizer Jun 29 '18 at 5:07
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Just a point of view not fitting into comments.

In a sense it has not been disproven, it has been superseded by the scientific demand of any model fitting experimental or observational evidence, and reduced to metaphysics.

After all a large number of people in the world still believe in the "evil eye". It is instinctive, even recently I stumbled and almost lost my balance, and immediately looked and saw a young man on his motorcycle stopped at the light looking at that grandmother running along, and he put his head down and went away.

In addition, it may be possible that our neural centers might be able to create an electromagnetic beam , after all superconductive circuits are proposed to model neuron activity! , which would follow the reverse path, with frequencies that are below optical, and register reflections. I am not saying it is so, but it is a model that would bring metaphysics into physics.

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Already Euclid did not believe the emission theory with the argument that light should take a finite time to reach the object and then travel back. Aristarchos had developed a heliocentric model and made a realistic estimate of the distance to the Sun. Since the stars are even much further away, how come that you see them immediately when you open your eyes? Since the measurement of the speed of light this argument becomes quantitative; it should take 16 minutes before you see the sun after you open your eyes. Of course, one could argue that celestial objects emit light themselves but then the emission theory really goes out the window. Allowing an eye to see light emitted from another object, not from itself, is the end of the idea. We should then see more light if more people are watching.

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  • $\begingroup$ The ancient Greeks thought that the Sun, lamps, the stars, etc. emitted light that hit the eye, but in the case of ordinary objects light goes from the eye to the object. $\endgroup$ – Keshav Srinivasan Jul 2 '18 at 18:33
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Science works on clearly stated theory with clear conditions and therefore clear statements of what those theories predict. These things may be subtle, and complicated, or require difficult math. But they are definite and clear.

The notion that something or other comes out of the eyes and makes vision possible somehow or other is just too vague to be called a scientific theory.

Cover one eye. The image the other eye sees is unaffected. This eye can't see any difference because of that eye. Similarly, you can't tell if somebody else is looking at a thing. Say you had an unknown number of people behind a wall, some number of which are looking at a test target. You can't tell how many are looking. Or if you have a bunch of people looking through telescopes from hidden vantage points. If you can't see the telescopes, you can't tell what they are looking at.

Oh, that's not how the eye-beams work. That does not disprove it. This eye is not affected by that eye because, somehow, this eye does not affect that eye.

You look at the moon, which is 1.3 light seconds away (very roughly) and blink your eyes randomly. You never open your eyes to not see the moon. You move your vision around in irregular fashion. You never move your eyes back past the moon to observe you can't see it. So, if something is coming from your eyes and going to the moon it has to be going fast enough so as to be less than your ability to resolve time, say something like a 30th of a second. So eye-beams would need to move at least 60 times the speed of light. You can similarly see Mars, even Jupiter, even stars. We know they are much farther away. And blinking or flipping your eyes does not ever result in you being unable to see that distant object. So eye-beams would have to move arbitrarily large speed.

Oh, THAT'S not how the eye-beams work. That does not disprove it. Eye-beams don't have to travel in normal ways, somehow.

Mirrors? Somehow. Lenses? Somehow. Diffraction gratings? Somehow. Fresnel lens? Somehow.

Somehow, somehow, somehow.

No. Just no. In order to test a theory the theory has to be definite, specific, and clear. So there is no need to disprove any such theory, since there is no such theory. There is only a dogged bit of poetry.

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    $\begingroup$ nevertheless it lead into geometry,en.wikipedia.org/wiki/… , at those ancient times , not just poetry, but euclidean. $\endgroup$ – anna v Jun 29 '18 at 4:22
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    $\begingroup$ As @annav says, the extramission theory of vision wasn’t just a bit of poetry, but a detailed mathematical theory. An incorrect theory, but a theory nonetheless. $\endgroup$ – Keshav Srinivasan Jun 29 '18 at 5:00
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    $\begingroup$ There once was a notion of eye-beams That was said to be how vision seams The objections were weak-manned Then ignored and ash-canned And science dispensed with as dreams $\endgroup$ – user93146 Jun 29 '18 at 13:13

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