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:
- Light shoots out of a person's eyes to an object and back (see the emission theory discussed above).
- A person's soul "interacts" with the things you see.
- 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.