Splitting Light

"In a very dark Chamber, at a round hole... made in the Shut of a Window, I placed a glass prism..." begins a chapter in Opticks, the book that describes Newton's experiments with light and colour in 1665. He worked with the hole described above to produce a stretched image of the sun with a blue top edge and a red lower edge. When the light passed through a split instead, he saw a multi-coloured band which we now refer to as the spectrum. He concluded that white light was made of many colours and noticed that the prism seemed to bend or refract light of different colours by different amounts allowing them to spread out and be seen.
Not only did Newton split white light but he also recombined it again and investigated the colours produced. His drawings show how he passed a beam of light through a prism then a lens. The lens made the colours converge on a second prism and the second prism spread the converging light rays so that they became parallel and formed a beam of white light on a screen. Newton also discovered that if he cut out or intercepted any of the coloured light that hit the lens this colour would disappear from the spectrum on the screen.
Newton also wrote about the way rainbows are formed in Opticks. Although the French philosopher Rene Descartes was the first to reveal the mystery of the rainbow, Newton was able to explain how refraction of light occurs when sunlight passes through raindrops.
A cut diamond acts like a set of prisms. When light passes through the diamond, the colours are dispersed and reflected back out. The angle of each facet is specially calculated to give the diamond its 'fire'.