Purple is not in the visible light spectrum, but neither is red, orange, yellow or any other color. Colors are phenomenal sensations. They only exist in our brains.

Colors that can be produced by visible light of a narrow band of wavelengths (monochromatic light) are called pure spectral colors. The various color ranges indicated in the illustration are an approximation: The spectrum is continuous, with no clear boundaries between one color and the next.

The visible light spectrum is the region of the electromagnetic spectrum that human eyes see. It runs from wavelength of about 400 nanometers (nm) at the violet end of the spectrum to around 700 nm at the red end of the spectrum.

Why does adding red light with blue light give purple light? Our eyes have sensitive rod cells that roughly act as our black and white vision, as well as less sensitive cone cells which distinguish color. Three sets of cones exist, responding to red, green, and blue light.