Recently, I was lucky enough to visit the mighty Victoria Falls. As I stood at the falls’ edge drenched in spray, I spotted double rainbows formed by sunlight being refracted through the water droplets.
One of the first people to explain how rainbows form was the Persian mathematician Kamal al-Din al-Farisi, who was born around 1260. Using a glass sphere filled with water to represent a raindrop, he showed that sunlight is bent as it enters the drop, reflects off the back of the drop, and is bent again on its way out. If rays are reflected twice inside the drop, a secondary rainbow is formed with the colours reversed. Here’s a more detailed explanation. Around the same time Theorodic of Freiberg performed a similar experiment. The two were not in contact, but both had been influenced by Ibn al-Haytham‘s Book of Optics. You can find out more about al-Farisi and al-Haytham in the 1001 Inventions exhibition.
Isaac Newton explained that the rainbow’s colours arise as a result of white light being split into its constituent colours. Many people will have childhood memories of making a Newton colour wheel with a disc of cardboard and a pencil. Here’s a late 19th century version.
As our understanding of the nature of light has continued to change, so has our understanding of the rainbow. For a detailed account of how people have portrayed rainbows in science and beyond, check out Raymond Lee and Alastair Fraser’s The Rainbow Bridge: Rainbows in Art, Myth and Science.