“Roy G. Biv” right? But if we could see into the UV and/or IR would there be two extra colors?
And some have said there really is no “indigo”- that the scientist that named the colors wanted 7. But how about the width of the bands? Is “indigo” as wide as others?
The scientist was Isaac Newton, who apparently was into numerology as well as alchemy, math, and physics. If we could see UV and/or IR, there’d still be 7 colors because Newton thought 7 was a magical number.
There are no boundaries on the colors of the rainbow so there’s no way to say any one band is wider or narrower than the others. If you get a nice bright one and examine it carefully, you can see more than 7 distinct colors while a faint one may have no more than hints of red, green, and blue.
I can’t distinguish more than 6 colors in a rainbow, myself. If you show me something and claim it’s indigo, I will think that it’s bluish purple or purplish blue. And what is the deal with calling stuff violet, anyway?
There are three primary colors (red, green, and blue) which correspond to the three frequencies of light that the cones in your retina are most sensitive to. Each color in the rainbow (i.e. ROYGBIV) is an arbitrary mix of those three primary colors. It’s simple to divide the spectrum into six colors-- three primary colors, and three secondary-- but you could divide them up into as many different combinations of RGB as you want, and name them whatever names you like.
There are indeed Infrared rainbows, and they have been photographed. For the reference, see Jearl D. Walker’s The Flying Circus of Physics. The first one I saw was published in Science or in Nature. There might even be a picture on the Web.
I haven’t heard about ultraviolet rainbows – water probanly absorbs any UV wavelengths. But if you used a UV-transparent liquid instead of water (like a fluorocarbon fluid) you ought to get UV rainbows as well.
The relative widths of the colors most emphatically do change with circumstances – the circumstance being the size of the droplet. M. Minnaert, in his book The Nature of Light and Color in the Open Air (orig. published by Dover; now in print under Springer-Verlag) even has a table that lets you determine the sizes of the raindrops making up the bow, based on the relative widths of the bands. This, by the way, is a potent argument that the rainbow is a diffractive phenomenon, not a strictly refractive phenomenon. If the appearance of the rainbow depended only upon the refraction of light, then the rainbow would look the same, regardless of the size of the drops. But diffractive effects become important as the drop size approaches the size of the wavelength of light. And, yes, you can roughly measure the width of a band, even though there is no sharp demarcation.
I’ve long suspected that “Indigo” has been retained as a color just so that “Roy G. Biv” has a last name with a vowel in it. Seven colors probably owes its existence to numerical mysticism, but that long predates Newton. See Boyer’s book The Rainbow from Myth to Mathematics. We do indeed seem to have three degrees of freedom in our color vision, but that’s peculiar to our visual system. I can easily imagine extraterrestrials with a quatrostimulus color vision, as opposed to our tristimulus system. Our color printing and color TV would look all wrong to them. The different colors of the rainbow are caused, in a sense, by the differing wavelengths of light. Our perception of them, on the other hand, is linked to that tristimulus response. But before you go off too sure of yourself, look up Edwin Land’s “retinex” theory of color perception.