The English mathematician and natural philosopher Sir Isaac Newton (1642-1727) made many important contributions to the study of physics, even apart from his famous laws of motion and gravity. It has been said that his studies in light alone would have placed him amongst the front rank of scientists.
About 1666 he passed sunlight through a triangular glass prism and obtained the spectrum of colours. The sunlight was dispersed (split up) by the prism into its component colours, spread out on the 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, Isaac 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 colours in the rainbow or spectrum.
Another experiment in the same field was carried out using a colour wheel. This is a disc, painted in the colours 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 disc apparently changes colour and becomes completely white. “White” light is thus shown to be made up of all colours in the rainbow.