Newton's visible spectrum

Newton's colours

Newton identified seven colours in the spectrum on his study wall:

Red, orange, yellow, green, blue, indigo and violet

In truth, a different number of colours might have been suggested (e.g. many people would not identify indigo as a separate colour), but at that time it was believed that the number seven had a mystical significance. Newton therefore proposed these seven colours.

Believing that white light was made up from the seven colours, Newton planned a crucial experiment to test his theory of differential refraction. This was very important because the accepted theory of the day suggested that the prism acted on any incident light to change its colour (to add colour), rather than that it involved a process of separation of colours by refraction. In this experiment he used two prisms.

The first prism produced a full spectrum of colours and Newton isolated narrow beams of light of a single colour from this spectrum and directed them at the second prism. For all of the single colour beams, Newton found that there was no further change of colour as they passed through the second prism.

When any one sort of Rays hath been well parted from those of other kinds, it hath afterwards obstinately retained its colour, not with standing my utmost endeavours to change it.

This finding provided evidence to counter the argument that the prism acts on the ray of light to change its colour. The finding supported Newton's view that sunlight, or white light, is made up from light of different colours, and that the prism acts to disperse those colours by refraction.