> Your brain has come up with a way to bring the ends of the visible spectrum together Not really your brain, more your eyes. Our eyes are not spectrographs:

> Your brain has come up with a way to bring the ends of the visible spectrum together Not really your brain, more your eyes. Our eyes are not spectrographs: they don't measure the level of every frequency of light. Instead, (most) humans are trichromats, meaning we have three types of color receptors (the cone cells). Each type has a different sensitivity curve. For trichromat humans, one type is most sensitive around red (~650nm), one is most sensitive around green (~550nm), and one is most sensitive around blue (~450nm). These sensitivity curves overlap, so photons between 650nm and 550nm will stimulate both the red and green cones. This means it's possible to simulate a single frequency between two adjacent types with two (or more) frequencies. For example, red light plus green light can also stimulate these cones in exactly the same way as true yellow (580nm), so there's no way for the human eye to distinguish between them. This is the basis of most display (and process color printing) technologies. An eye with more types of color receptors would not be fooled as easily. Magenta is what we perceive when red and blue cones are both stimulated simultaneously in a way that is not possible from a single wavelength, as their sensitivity curves don't overlap. So when you see magenta, you're seeing at least two different wavelengths of light. With hues not between red and blue on the color wheel, it's possible that there's only one wavelength responsible (but it's also possible that it's a combination of wavelengths). this is an oversimplification -- blue is actually completely overlapped by the other two cone types, but they have very low sensitivity in most of the blue range.