Why doesn’t it seem to be a smooth fade from violet to red, as a fade from white to black appears as one continuous fade along an infinite number of greys? I know that white to black involves a change in amplitude, whereas violet to red involves a change in frequency, but taking another stab, the sound of a smooth frequency fade from 110 Hz to 220 Hz has a smooth change, similar to the look of white to black.
Also, why do there seem to be 7 colors, instead of say 13?
Is all this related to perception, i.e. either the physical nature of our eyes or the wiring in our brain? or is it just a natural state of color, which is likely perceived similarly by gorillas or bumble bees, or aliens from Mars?
Surprisingly, this is culturally determined. Americans, famously, see only six colours. It has been suggested to me that the writings of earlier civilisation suggest that they saw three or four colours, but I have no cite for this.
In Irish, there are three different words used for the colour green. Two of them are translated into English as “green”, and one as “green” or “grey”, depending on the context. To an Irish speaker, they are not interchangeable. Irish speakers see the different shades of greeen as comprising three distinct colours, one of which overlaps with grey.
(For the record, “liath” is a grey-green, “glas” is a blue-green and “uaine” is a bright yellow-green.)
What this suggests to me is that perceptions of different colours, and the boundaries between different colours, are matters of culture and not physics.
We don’t perceive 7 colours. We can perfectly tell apart much more colors than that, and even have names for them. And finally, there’s a smooth fade from violet to red. The visible spectrum was arbitrarily divided in 7 colors (how often do you describe something as being “indigo”?), IIRC to match the 7 notes in music.
Probably had something to do with the Ancient Greeks - Pythagoras and his crowd loved numbers. Seven was special (actually a lot of numbers were special) for mathematical reasons and also there were seven objects in the sky (Sun, moon and five visible planets). This led to seven notes in the musical scale, the “Harmony of the Spheres” and other mathematical mysticism. Part of that was the seven colours in the rainbow, which is a bit of a stretch, I mean are there really two distinct shades of purple??
For us colour blind people, we (or I) see a dirty yellow, and a greenish blue. Period.
I’m pretty sure people are misunderstanding the OP, although I’m not sure I can explain it better.
When you have a smooth transition from black to white, there’s gray in the middle. We consider gray halfway between black and white. When you have a smooth transition from red to violet, there’s reddish-violet in the middle. A rainbow does not do this. In the middle is green, something we do not consider halfway between red and violet.
In the case of sunlight, the colors are affected by the Earth’s atmosphere’s opacity. The atmosphere is not equally transparent for all frequencies in the visible range. Also, our eyes are more responsive to some colors than others. Possibly the absorption lines in the sun’s spectrum may affect our perception as well.
In the case of a “pure” white light passing through a prism, I would guess that our eyes’ sensitivity to various colors is the primary factor.
Thanks, Achernar for the excellent clarification. Thinking further on this has only brought me further questions:
Why is it that mixing red light and yellow light makes orange light? In music, if you mix an C note and an E note, you don’t get a D note; instead you get a C note and an E note.
Even more curious, why is it that mixing red light and blue light makes violet light? At least red and yellow are close on the spectrum, with orange in between them. not so for red and blue.
Outside our spectrum, does this “banding effect” exist as well? We just talk about “ultra-violet” and “infra-red”, but if we could see them, would they also appear as a spectrum? Can various infra-red frequencies be mixed together to make other colors as in our visual spectrum?
The more I think about this, the more I think it must be perception; there’s no physical reason I can think of that these 7 bands exist, or that they mix to create other other colors.
Your eyes have three different kinds of cones. They’re each responsive to different wavelengths (colors) of light, and they each respond to a variety of colors, but they’re most responsive at certain wavelengths. Call them Red, Green, and Blue cones. This is why your TV and computer monitor have red, green, and blue phosphors - so they can stimulate the cones in your eye separately.
But that’s not how your brain perceives color. The stimuli from Red and Green cones are paired off and weighed against each other, and the same happens to Blue and (Red+Green). This gives you four primary colors, perceived as two pairs of opposites: red<—>green, and blue<—>yellow. That’s why yellow on your TV screen (which is really red and green together) looks yellow instead of reddish-green - the red and green cancel out on the first spectrum, so you don’t see either the red or the green, but you perceive the yellow because there’s no blue to cancel it out.
That’s also why red light plus yellow light makes orange light. If your red cones are stimulated twice as much as your green cones, you’ll perceive both red and yellow, which your brain calls orange. That can happen whether you’re seeing two colors of light combined on a TV, or whether you’re seeing a single wavelength of light (from a laser or prism) that happens to stimulate red cones twice as much as green cones.
NOt sure if it goes back that far, but the idea of there being seven distinct colors in teh rainbow goes back at least as far as Newton. He ‘chose’ seven (if chose is the right word) because there are seven different notes in a musical scale, and it had previously been shown that the most pleasing combinations of notes could be represented with simple fractions (any combination of two notes can be shown with a fraction, it’s just a matter of 2/3 versus 21/9). Anyways, he decided on seven because coupled with music, it proved, for him at least, the rational order of the universe, or something like that.
Granted, I’m no expert. This is just what my teachers have been tellings me. Hey, I thought I was asleep through those classes. Guess not