Wavelength and Color
We are told that wavelength determines color. But what does “determine” mean here? It is not in doubt that the wavelength of incoming light causes the perception of a specific color, though we should note that it only does so in conjunction with non-trivial facts about the perceiver’s eyes and nervous system. This is compatible with allowing that other types of stimulus could causally determine color vision: for example, the wavelength of sound waves might conceivably cause colors to be seen, or indeed a knock on the head. The causal relation is quite contingent and extrinsic with respect to the nature of color. What we know is that wavelengths are causally correlated with the perception of particular colors, but this is a far cry from supposing that they constitute what colors are. Kicks to the shins are causally correlated with bruises of various hues, but no one would suppose that bruises are kicks.
Is there any closer relation? It might be supposed that an identity theory is possible: that way we would have a nice materialist account of what color properties are. It is worth being very clear about what this would mean, which requires us to be explicit about what wavelength is. According to the wave theory of light, light has a wavelike structure analogous to the waves you can see in the ocean: waves have both frequency (how many cycles reach a certain point in space per unit of time) measured in Hertz units, and they have length (the distance between successive crests or troughs) measured in meters. Wavelengths are very short for light waves and hence are measured in nanometers. The visible spectrum (between 400 and 700 nanometers) is just one part of the much wider electromagnetic spectrum. Thus the identity claim is that a given color can be identified with waves of light with a specific distance between incoming peaks—this being nothing other than the phenomenon you can observe on many a beach (though involving a light medium not a water medium). So the contention is that colors have a wavelike structure, travel through space, and have specific distances between their wave crests: red, say, is shaped like a wave, traverses space, and has a certain wavelength (longer than other colors). These are attributes of light, so by Leibniz’s law they must be attributes of color, if colors are identical to light waves. That is the nature of the color red, its true character, its inner essence: to be red is to have a particular wavelength, i.e. a certain distance between successive crests.
But is this the way red looks? If you stand on a beach and look out to sea, you will see waves rolling into shore, and these waves will look to have a specific wavelength, generally of several meters: the shape and height of the waves is apparent to you and the distance between them is also apparent. These waves look precisely like the waves they are. But do colors look like waves? Evidently not: when you look at a red apple you don’t have an impression of waves of light approaching your eyes—you don’t see anything wavelike at all. You don’t see crests and troughs, or wave heights and the distances that separate them. The phenomenology of seeing colors does not include any perceiving of wavelike structure—you don’t, say, see red as having a longer wavelength than blue. Why not if colors just are wavelengths of light? You might say these waves are too small to see, that’s why you don’t see them—but they are still what visible color actually is. But if red were made of much bigger waves would we see them when we looked at a red object? Would red look like the waves it is if the waves were big enough to see? Clearly not: it would presumably look something like colorless ocean waves. And if we looked through a microscope and could see our actual light waves, would we still be seeing red? No, the redness would disappear, to be replaced by magnified waves of light. So colors don’t appear to be wavelengths, as we ordinarily perceive colors—yet we are seeing some sort of property. The property we see cannot then be the property of being a wave with a specific wavelength.  Some philosophers will respond by saying that these color appearances are actually appearances of light waves with their corresponding wavelengths, with no further property interposed between them and the perceiver. But this is a bad theory of perception: we clearly do perceive color properties when we see colored objects–we see things as red, blue, green, etc. But the property that we see can’t be identified with a wavelength property, because it looks nothing like a wavelength property. At the very least we would need to be given an explanation for why colors don’t look the way they intrinsically are, according to the identity theory; but no such explanation is forthcoming. So the phenomenological unreality of the alleged wavelike nature of color is a count against the theory that colors are identical with such properties.  And it isn’t as if the way colors look gives us a hint of their real wavelike nature: there is nothing wavelike about the way color looks to us. If anything, colors look non-wavelike. We could, of course, go eliminative about colors, claiming that colors don’t really exist as properties, though light waves do—then there would be nothing that is not covered by the physics of color. But if we insist instead that colors are real and also reducible to wavelengths, then we have to face the question of why they don’t seem that way. On the face of it, the identity theory of color is refuted by the appearances. To be sure, wavelengths trigger the perception of color properties—they are the external cause of color perception—but they are not what color properties are. These properties are ontologically separate, a distinct realm of existence.
There isn’t even a supervenience relation here, since the same wavelengths could correlate with distinct colors in different possible worlds, depending on the eyes that respond to these wavelengths. The correlation in the actual world is just that—a contingent correlation. Waves in the ocean often correlate with gleaming peaks and a crashing sound, but it would be quite wrong to think that there is some necessary connection here; a trip to a nearby possible world would quickly disabuse you of that misconception (in that world ocean waves form only at night and merely murmur). We must not mistake correlation for identity. The relationship between wavelengths and colors is an external contingent relationship not a relationship of identity or constitution. The reason colors don’t look like waves of light, equipped with frequency and wavelength, is that this is not what they are. The same can be said of heard sounds and sound waves in the atmosphere: pitches don’t sound like waves with frequency and wavelength because they are not such waves. We can hear waves in the ocean and gauge their frequency and wavelength, but nothing analogous is true of heard sounds themselves—they are not phenomenologically wavelike. Sound waves cause sound perceptions, certainly, but the sounds heard are not identical to sound waves in the atmosphere: one doesn’t hear middle C as a particular spatial separation of wave crests. Sensible qualities like these are not reducible to wave patterns in a physical medium.
 A further point: if colors are identical to wavelength formations, which are invisible to the human eye, how come we can distinguish colors from each other? How can we tell that red is not the same as blue if we can’t see the wavelength difference that constitutes the difference of color? The obvious answer is that the property we see when we see red is not the same as the property of having a certain wavelength but is a property that is entirely visible to the human eye.
 We may note, too, that wavelength is a continuous quantitative property, being simply distance between two points, but colors vary qualitatively and are discontinuous: red is not simply “longer” than blue. So wavelength is unable to capture the way the visible spectrum is divided up. There ought to be just one color, varying along a single dimension, if colors were definable in terms of wavelength.