Colour is actually produced by light as it is broken down into electromagnetic vibrations. Variations of the wave lengths of the vibrations cause the viewer to see different colours. the longest wavelength is perceived as red, the shortest as violet. Ancient Greek philosophers asserted that colour is not a physical property but a matter of perception, but it was not until 1666 that Issac Newton produced scientific evidence of that fact. As an experiment, Newton directed a beam of sunlight into a glass prism. Since glass is denser than air, the light was bent as it passed through the prism. Newton expected this but he did not expect the light to be dispersed into colours as it left the prism. As the waves of light emerged from the prism they arranged themselves into the colours of the rainbow: indigo, blue, green, yellow, orange, and red. There is no purple in the natural spectrum of colour, but where the first (indigo) and the last (red) colours overlap, the result is purple or violet. Newton joined the colours into a circle so the flow from tone to tone would be continuous, therefore creating the first colour wheel.