In a sense, color is an optical illusion. Human perception of color is a strange and wonderful thing, created in our brains from the variation of the wavelengths of light that reach our eyes. If you remained awake during high-school science class, you'll recall that the retina of the eye contains rod cells (which are used for detail and black-and-white vision when there is not much light) and three types of cone cells, which respond to a different wavelength of light, all in the 400 nanometer (violet) to 700 nanometer (red) range of what we call the color spectrum.
All the colors we could see (if our eyes were constructed that way) would reside in that continuous color spectrum, like the one shown in Figure 6.1. However, we can't directly sense each of those individual colors. Instead, each of the three kinds of cone cells in our eyes "see" a different set of frequencies, which happen to correspond to what we call red, green, and blue (RGB). Our brains process this RGB information and translate it into a distinct color that we perceive.
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