In the mid-1660s, Isaac Newton bought a pair of prisms at a fair near Cambridge, which were to be the basis of a series of experiments that would unlock a secret that had occupied scientists for centuries - the nature of light itself.To explain what he had done, Newton created a diagram. It is called The Crucial Experiment and is a pivotal image in scientific history, a graphic moment when the ancient world was overturned by modern science. Newton demonstrated that white light is not pure, but made up of a number of different colours, the colours of the rainbow.Newton's ideas transformed our knowledge of what we see and how we see, and the prism and its refracted colours became a captivating image. From fibre-optics to the cover of Pink Floyd's Dark Side of the Moon album, Newton's work went on to influence centuries of science and art.