Rainbows are fascinating. There is something simply magical, when you look up at the sky and sea their incredible colors. And although humans have marveled at them for thousands or even millions of years, it wasn’t until 1637 and the French scientist René Descartes that they were finally able to understand them. In his essay “Dioptrics” he was able to derive a law, describing refractions – the way light behaves when crossing the border between two different media, like for example water and air. Which is exactly what happens when rainbows are formed. The rays from the sun pass through tiny water droplets in the air and are reflected back from their rear surface. Whenever they pass between the two, they are refracted or bent slightly. However, the different wavelengths, or colors, that make up the light are bent by different portions. This causes the colors to spread into the beautiful fan that we are used to.
Because the light needs to be reflected back of the raindrops before it hits our eyes, it is only possible to see a rainbow with the sun standing behind us and due to the 42-degree angle, at which water refracts light, this is only possible if it is sitting low, close to the horizon. This is why we can only ever see a rainbow as a semi-circle. But if we could in some way be between the drops and the sun, while it is higher, like for example in an airplane, we would observe a complete circle.
A similar phenomenon is what is known as Halos. These are another type of circular rainbows, but they are formed by ice crystals in the atmosphere. For this to happen, a person would need to be either close to the poles, where it is colder, or again at a very high altitude. However, the ice refracts the light at a much smaller angle – 22 degrees, so full circles are actually observable from the surface.
Incredibly, throughout the years there has been much debate about how many colors actually make up a rainbow. According to the Greek poet Homer, there was only one color and that was purple. This claim was disputed by the philosopher Xenophanes, who declared there are two additional colors – red and greenish-yellow. During the Renaissance it was believed that the colors are actually four and it was Descartes himself, who added a fifth one, making them red, yellow, green, blue, and purple. Around 30 years later Sir Isaac Newton included indigo and orange and today these seven colors are perceived as the seven colors of the rainbow. However, as there is no hard boundary between them, it is very much up to interpretation.