Isaac Newton graduated from Cambridge University's Trinity College in 1665, the year that the Great Plague struck London, and like many others, he abandoned the city. He returned to his family's farm in the countryside hamlet of Woolsthorpe-by-Colsterworth. Divorced from his usual pursuits, Newton entertained himself by exploring the nature of color.
Newton was, of course, a multifaceted genius, who created the foundation for classical physics in his Principia Mathematica. In that monumental work, he gave us the three laws of motion and the law of universal gravitation. Newton is credited with developing calculus along with Gottfried Liebniz (with whom he had a bitter rivalry) along with many other mathematical tools, like the generalized binomial theorem. But optics was Newton's first love.
By the seventeenth century, the technology of optics was already a well-developed field; high quality microscopes had been built by Robert Hooke, Antonie van Leeuwenhoek and others. The refraction of sunlight into colors by a prism had been observed but was not understood. It was generally thought that the 'pure' white light was contaminated by 'gross matter' to yield colors.
Newton began his investigations by cutting a pinhole in his window shade to let in sunlight, which showed up on his wall as a round illuminated area. Refracted by a prism, it turned into an oblong area with a rainbow of colors. Intrigued by the change in shape, Newton cut a variety of holes of different sizes and shapes, but no matter the shape of the original beam, the refracted light turned more oblong.
Newton also placed a second prism of the same type in the path of the light and was able to turn the colors back into white light. This showed that white light, rather than being pure, was composed of a miscellany of colors.
Newton's crucial experiment was to refract light onto a piece of wood, into which had been drilled a small hole. In this way, he was able to obtain a beam of light with a pure color. He was able to show that blue light, for instance, when refracted through a second prism yielded again only blue light. Red light yielded only red light. Moreover, the angle at which light was deflected onto his wall was dependent on the color. Different colors of light had different degrees of "refrangibility" (to use Newton's term), which were an inherent property of that color.
A thin slice of gold leaf reflects gold light from a candle, Newton found, but appears blue if viewed from behind. The colors of the world are not revealed by light, but come from light itself, while objects are "variously qualified to reflect one sort of light in greater plenty than another."
Newton was notoriously slow to publish, and his New Theory of Light and Colors did not appear in the Philosophical Transactions of the Royal Society until 1672. The publication set off a feud with Robert Hooke, a powerful man in the Society, who had his own theories about color. Though the community of scientists in the seventeenth century was a small one, it was no less contentious than it is today.
Newton thought that light was composed of extremely subtle "corpuscles," an idea reflected in the division of light into photons today. His use of multiple prism arrays, described in his Opticks, published in 1702, were arguably some of the initial experiments that led eventually to the development of tunable lasers.