Spectrum
The band of colours, red through violet, into which white light can be spread.
Also: visible spectrum, rainbow colours
Pass sunlight through a glass prism and it fans out into a band of colour: red, orange, yellow, green, blue, violet. This is the visible spectrum, light sorted by wavelength. It is a small part of a much wider range that includes radio waves, infrared, ultraviolet and X-rays, none of which we can see. The colours merge smoothly; the familiar list of seven goes back to Isaac Newton, who added indigo, perhaps to match the seven notes of a musical scale. A rainbow is the same spectrum made by raindrops.