Visible light ranges from violet (400 nanometers wavelength) to red (700 nanometers wavelength). Why then does one think about the primary colors as being Red, Green and Blue. In fact painters have known for years that they can combine these to produce all kinds of shades. The answer, originally given by Helmholtz is that our eyes (its cones actually) have only three different kinds of receptors with sensitivities around R, G and B. Thus, we project the infinite spectral content in the visible spectrum into components centered about R, G, B. In the language of filtering, three types of cones are bandpass filters centered about R, G and B.