Do infrared and ultraviolet bands exist in rainbows?

[CalMeacham]

@Chronos

Does glass opacity start at the same wavelength as the eye’s opacity? I’d expect there to be at least some margin between the two.

There is, and it cepends upon both the type of glass and the wavelengths that you designate “ultraviolet”. In general, though, glasses cut out somewhere between 300 and 400 nm. The eye’s response is getting pretty low by 400 nm, but you can definitely see lines down there (the Hg line at 404 and the Hydrogen Balmer line nearby) ( Infrared basics: definitions and detectors )
I have to look more closely into Ritter’s work. Internet sources say that he used a glass prism, but I’m suspicious. It was a bit too early for diffraction gratings. Rittenhouse had constructed the first consciously made diffraction grating in the previous century, but never actually measured wavelengths with it. Thomas Young, the champion of the Wave Theory, used an instrument maker’s finely ruled graticule as a grating to perform the first-ever wavelength measurement, but neither IR nor UV light had been discovered yet. So Ritter definitely used a prism.

[jtur88]

Then, are infrared and ultraviolet “primary colors”? Why not? Is the concept of “primary colors” a human perception construct, or must there be exactly three primary colors for scientifically explainable reasons?

[Derleth]

@jtur88

Then, are infrared and ultraviolet “primary colors”? Why not? Is the concept of “primary colors” a human perception construct, or must there be exactly three primary colors for scientifically explainable reasons?

The concept of “primary color” is rooted in human biology, but imperfectly: As opposed to what some of us learned in grade school, the three kinds of cone cells, which are responsible for color vision, don’t map neatly to red, green, and blue, but instead have light sensitivities which peak at 420 nm (for S-type cells (Short wavelength)), 530 nm (M-type cells (Medium wavelength)), and 560 nm (L-type cells (Long wavelength)). Nevertheless, we do have three distinct kinds of cone cells, so we have three primary colors, which we can mix in equal proportion to get a color which is perceptually white to human beings.

So primary colors aren’t entirely arbitrary, but neither are they a perfect representation of human biology, and they’re certainly nothing to do with any intrinsic property of electromagnetic radiation.

[BigT]

What’s interesting to me (and still IMO not well explained) is how our brain/biology turns it into a color wheel. There’s no inherent reason that red+blue should look like violet.

[dtilque]

@BigT

What’s interesting to me (and still IMO not well explained) is how our brain/biology turns it into a color wheel. There’s no inherent reason that red+blue should look like violet.

From here

The erythropsin in the red-sensitive cones is sensitive to two ranges of wavelengths. The major range is between 500 nm and 760 nm, peaking at 600 nm. This includes green, yellow, orange, and red light. The minor range is between 380 nm and 450 nm, peaking at 420 nm. This includes violet and some blue. The minor range is what makes the hues appear to form a circle instead of a straight line.