Newton's color disk
The English mathematician and natural philosopher Isaac Newton made many important contributions to the study of physics, even apart from his famous laws of motion and law of gravity. It has been said that his study of light alone would have placed him among the front rank of scientists.
In about 1666 Newton passed sunlight through a triangular glass prism and obtained a spectrum of colors. The sunlight was dispersed (split up) by the prism into its component colors, and spread out on paper.
This is the same effect as in the rainbow (where the raindrops act as prisms). Although theories of the rainbow had been put forward at least half a century before this, Newton cleared up the subject by passing the spectrum back through another prism and producing white light once more. This was final proof that white light is made up of all the colors of the rainbow or spectrum.
Another experiment in the same field was carried out using a color wheel (see illustration). This is a disk, painted in the colors of the spectrum, which can be spun round rapidly by turning a handle. The somewhat surprising result is that when it is turned rapidly, the disk apparently changes color and becomes practically white.