Magenta Has No Single Wavelength
A rainbow spreads light into its component wavelengths, from red through orange, yellow, green, blue, and violet. Each hue along that band corresponds to light in a particular part of the visible spectrum. Magenta has no place on that continuous wavelength scale: there is no single wavelength of light that produces it.
That does not mean magenta is imaginary or that magenta-colored objects cannot be seen. It means the sensation is not tied to one isolated spectral wavelength. A screen can make magenta by combining red and blue light, and pigments can reflect a mix of longer- and shorter-wavelength light while absorbing much of the light between them.
The Brain Fills the Gap
The eye has cone cells that respond to overlapping ranges of wavelengths, roughly associated with long, medium, and short wavelengths. The brain compares their signals rather than reading off a hue from one cone alone. When red- and blue-sensitive signals are strong together, with relatively little signal from the middle range, the visual system interprets the combination as magenta.
On a spectrum diagram, red and violet sit at opposite ends. Magenta is the perceptual bridge the brain supplies when those ends are stimulated together; the spectrum itself does not loop around from violet back to red. This is why magenta is called a non-spectral color: it is a real color experience, but not one produced by a single wavelength.
The same principle explains why a display can show vivid colors that no rainbow contains. Its tiny red, green, and blue lights do not need to emit every wavelength corresponding to every color we perceive. They only need to create cone signals that the brain reads as those colors.
