Newtown noticed the gradual change of colours in the spectrum, however divided it into seven (red, orange, yellow, green, blue, indigo and violet) referring to seven notes of the major musical scale and the mystical power of the number 7 in Ancient Greece (McLaren, 1985; Newton, 1704). Indigo has been removed from the classical set of hues since then, due to its indistinguishability from blue and violet (Hunt & Pointer, 2011). In a way, dividing the spectrum to 7 seems artibrary.
Modern colour vision theory is based on trichromacy at the photoreceptor level and opponency from ganglion cells onwards. Considering that three opponent channels are black-white, blue-yellow, red-green, one could argue that there should be 8 hues (blue, yellow, red, green, and four other hues that are in between). However, traditionally there is no hue between yellow and green. Coincidentally, we end up with number 7 again. On the other side, one could identify several other specteal colours within the rainbow. This would require a process of linguistic explanation (colour naming), which comes after colour vision and hue categorisation. While explananing the rainbow (not primary colours) requires mostly cognitive psychology, colour naming is a lot more clustered with various discussions of universal, cultural, and individual influences.
thank you Alp! I thought that a logarithmic scale reprezentation of 380nm-740nm should be used and then there shurely were not exactly 7 colours to define.
Since wavelength is a continuous variable, there are infinite hues within visible light, and of course that is not taking into account combinations (which cannot be a feature of a single particle). As Alp Durmus mentions above, categorizing wavelength into specific types by color involves psychological and linguistic factors. In English it has been suggested that there are 11 unique color names: black, white, red, green, yellow, blue, pink, gray, brown, orange and purple. Not all of these would be posible with a single wavelength and some (orange and brown) could be created with the same wavelength, but at any rate there is nothing inherently special about 7 colors (or 11 for that matter).
dear John, I agree with you! (by the way: what about deep purple, pink floyd, green day, aso :))
Alp and John have given good answers, but let me also offer a few historical quotations that relate to the visible spectrum and trichromacy. All of these quotes are from: MacAdam DL. 1970. Sources of Color Science. The MIT Press: Cambridge, MA.
“. . . the rainbow appears with three colors. The rainbow has three colors, and these three an no others.”
Aristotle, 350 BC
“From three simple sensations … the different proportions in which they may be combined afford a variety of tints beyond all calculation. . .”
Thomas Young, 1845
“The theory which I adopt assumes the existence of three elementary sensations by combination of which all the actual sensations of color are produced. . .”
James Clerk Maxwell, 1856
“When we speak of reducing the colors to three fundamental colors, this must be understood in a subjective sense as being an attempt to trace the color sensations to three fundamental sensations.”
Herman von Helmholtz, 1866
Informally, these quotes suggest a transition in thinking from color as a property of light, to color as a property of vision.
Paul, while I do agree that there is something particularly special about trichromacy, I do think (and color opponency suggests) that the eye/brain does "see" more than those three primary colors. Nobody calls yellow, for example, a mixture of red and green, even when it is. Of course in color opponent theory yellow is promoted to a fundamental color. But even still it often seems 'natural' to consider a color like 'purple' as its own color, or brown, or even pink. Few people would call brown a shade of 'dark yellow' or 'dark brown.' Been thinking about this a lot recently because I brought an RGB color mixing box to our Cub Scout meeting and enjoyed freaking the boys out with mixing red and green to make yellow instead of brown or gray, and then mixing yellow and blue to make white, not green. Seems like young children would make excellent subjects for a lot of color perception studies!
John, thank you! another question: how true is it, that, case the eye sees merely the reflected colour(s), a green leaf is everything but green (while green isn`t absorbed)?
- Badges
- Science topic