That we see things seems unquestionably obvious. But, when I try to think about how, I find it as a mystery.

The conventional answer nowadays to this question revolves around absorption and reflection of waves of some specific wavelength. And this is established long back. But, this theory seem not really natural from a more basic perspective.

Were there other theories explaining vision? What was the historical development of theories of vision?

  • $\begingroup$ Which one/both of these historic theories truly explains the science behind "seeing things" ? Where there any other theories challenging them in the past ? $\endgroup$
    – Amit Tyagi
    Nov 7, 2014 at 19:36
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    $\begingroup$ On my opinion, this is a wrong forum for this question. This site is for the history of science not for the discussion of how things work or how they are. $\endgroup$ Nov 7, 2014 at 20:23
  • $\begingroup$ Thanks. I will deleted a couple my comments. Since we have quite a some already here. $\endgroup$
    – quid
    Nov 7, 2014 at 20:39
  • $\begingroup$ Thanks Omen for editing. @Alexandre do you still feel its not about history of science. $\endgroup$
    – Amit Tyagi
    Nov 7, 2014 at 21:31
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    $\begingroup$ As it stands now, I think this is a good question for this site. The answer given is exactly what this site is about. $\endgroup$
    – Joe
    Nov 8, 2014 at 4:45

2 Answers 2


As per your comment:

Were there any other theories challenging them in the past?

This topic was briefly discussed on Neil deGrasse Tyson's reincarnation of the TV show Cosmos. While I obviously can't insert the episode here (and can't recall which one it was, anyway), I can explain the basic premises of the early alternate theories, from other sources.

As usual, I'll get Wikipedia out of the way first. The idea I alluded to above is the emission theory, which stated that people emit light from their eyes, which bounces off objects and back to their eyes. It was first proposed in ancient Greece, and became a leading theory. The evidence for it, though, was extremely poor. Wikipedia also talks briefly about a second theory, which I'm actually unable to explain, because it doesn't have anything in its favor:

The second school advocated the so-called 'intro-mission' approach which sees vision as coming from something entering the eyes representative of the object.

Again, though, there is virtually no evidence in favor of either idea, so that about wraps it up for the ancient Greeks. Oh, but as a parting shot:

Winer et al. (2002) have found recent evidence that as many as 50% of American college students believe in emission theory.

Which is from here. Honestly, sometimes humanity can be so . . . I don't even want to say it.

I found a second discussion of it here. It goes along lines similar to the Wikipedia articles:

A theory of vision must explain how information crosses the space between the perceiver and the object he/she is looking at. Early theories of sight offered three major opinions on this matter. One opinion held that the eye sends out rays to objects and that these rays give the viewer information about color and shape. Among the best known proponents of this view were the Pythagoreans, adherents to the religious and scientific philosophy developed by Pythagoras (he of the theorem). A second opinion held that sight depended upon an interaction between images that were ejected from the eye and the perceiver's own spirit, or soul. Socrates and Plato were among the best known of this theory's supporters. Finally, another opinion held that when people see, they actually make contact with the objects they see, or with replicas of those objects.

To summarize, there were three theories:

  1. Light shoots out of a person's eyes to an object and back (see the emission theory discussed above).
  2. A person's soul "interacts" with the things you see.
  3. A person "makes contact" with things they see.

Not to be outdone, Democritus proposed a fourth theory:

In the fifth century B.C., the Greek philospher Democritos believed that every object contained numerous replicas of itself. These replicas were called eidola (singular is eidolon). Every object continuously emits eidola. If someone looks at some object, its replicas fly toward the viewer and enter his/her eyes. According to this scheme, you are able to see some object only because one of the object's eidola has winged its way into your eye. Several observations were taken as support of the eidolon theory. For instance, the fact that someone could see his/her reflection in water or in a mirror proved that the viewer is emitting eidola.

Like the others, though, it had flaws:

Here are two of the arguments that contributed to the theory's ultimate rejection (I've excerpted these from V Ronchi's book, The Nature of Light: An Historical Survey, Cambridge: Harvard University Press, 1970).

Suppose that you are watching a parade. The eidolon theory suggests that you see the marchers because each marcher's eidolon reaches your eyes. What, though, governs the relative positions of all the eidola? How do the eidola maintain themselves in a formation that corresponds to the parade formation?

According to eidolon theory, you can see your reflection because one of your own eidola struck the mirror and returned to your eye. But if each eidolon was flying away from you, it should fly with its "back" toward you; how does the eidolon contort itself so not only do you see its front but also so that in the mirror your right ear appears toward your right and your left ear toward your left?

These were the earliest theories about sight. Returning to your comment:

Which one/both of these historic theories truly explains the science behind "seeing things"?

Both do. These aren't theories of light; they're theories of physics. I suggest you take a look at the Wikipedia article on wave-particle duality for some more good information.

  • $\begingroup$ Why was this downvoted?? $\endgroup$
    – HDE 226868
    Nov 7, 2014 at 20:24
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    $\begingroup$ Not sure, I have upvoted, it is a full comprehensive answer. $\endgroup$
    – user22
    Nov 7, 2014 at 20:34
  • $\begingroup$ The article by Sekuler which you pasted is full of errors. For example quoting "Socrates" as a source, or claiming that "before Alhazen, people believed that stars couldn't be seen in the daytime because the stars had actually been extinguished". This man obviously knows nothing about the history of science. $\endgroup$
    – fdb
    Dec 22, 2014 at 10:49
  • $\begingroup$ @fdb While I fail to see why Socrates is an improper source, or why the Alhazen detail is false (I interpreted it as meaning that Alhazen proved that such a belief was false, not that it was necessarily believed by everyone; besides, it is irrelevant to this discussion), and while I disagree with the generalization that "This man obviously knows nothing about the history of science", I will certainly remove this section if you can show that it is false, and I appreciate that you voiced this concern (I apologize for the awkward phrasing; I couldn't come up with anything better). $\endgroup$
    – HDE 226868
    Dec 22, 2014 at 22:51
  • $\begingroup$ The "Socrates" in Plato's dialogues is a fictitious mouthpiece for Plato's own views. We do not know what the historical Socrates thought about this or any other question. To talk about "Socrates and Plato" as expounding some view or another betrays gross historical ignorance. $\endgroup$
    – fdb
    Dec 22, 2014 at 23:02

We see objects because objects emit their own light. How? The electrons which orbit inside the atoms which comprise the object interact with ambient light radiation. This causes the electrons to switch to higher orbits. Then, inexplicably, the electrons return to their original orbit, and in that process emit photons which our eyes detect. Thus Democritus had the best explanation. Now science hasn't figured out why the electrons return to their original orbit after they attain the higher orbit when the electrons interact with ambient radiation. However, words like absorption and reflection are misleading, just like the words dark matter have no meaning, as science doesn't know what dark matter is.

  • 2
    $\begingroup$ You seem to be making a lot of statements that would need very good references and elaboration to be backed up properly. In this post, you are contradicting much of the physics of the 20th century. $\endgroup$
    – Danu
    Dec 21, 2014 at 10:36
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    $\begingroup$ How did democritus know about electrons? $\endgroup$
    – Jasser
    Dec 21, 2014 at 12:34
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    $\begingroup$ The best example is different colors of paint. The molecular structure of blue paint is different than red paint. So when your flashlight shines on blue paint, it is the molecular structure of the blue paint that determines the frequency of light that the paint's electrons emit after they relinquish the higher orbit they achieved when they interacted with the flashlight. Objects emit their own light. Light doesn't reflect. Light energizes an object's electrons to emit their own photons informing us of the shape and color of any object. Source? I deduced this by myself in 2002. $\endgroup$
    – JoeFuture
    Dec 21, 2014 at 12:55
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    $\begingroup$ Democritus claimed that every object continuously emits eidola. Eidola is just a word, like dark matter. Democritus had the best description of the alternative theories presented above as to why we see things. Light doesn't reflect off of an object. The object doesn't absorb light and then reflect some filtered remnant of that absorbed light. No, objects emit their own light produced by their own electrons. This is how a proper explanation of why the sky is blue can be formed. The molecular structure of the gasses in the atmosphere produce blue light when sunlight affects their electrons $\endgroup$
    – JoeFuture
    Dec 21, 2014 at 18:41
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    $\begingroup$ It will be very nice of you if you also include the source of your information i.e., references. @JoeFuture $\endgroup$
    – Jasser
    Dec 22, 2014 at 13:34

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