It is amazing to me that many U.S. schools and websites teach one of Isaac Newton’s greatest mistakes as though it were a fact.
Newton proposed that there are seven colors in the rainbow. But he was wrong. It is neither true of the physical world nor of the human perception of it.
Around the world and at different times in history, people have seen/imagined anywhere from two to seven colors in a rainbow. The difference is partly human perception, and partly cultural, in that it matters what color words our language uses, and also it matters what we have been taught. (See for example this article on the matter.)
But in fact, our eyes see far more: scientific investigation reveals that we can distinguish many more colors than seven when a rainbow is sufficiently bright, crisp, and spread out. Here’s a photo of sunlight passing through a prism — its colors are themselves limited by the effects of the camera and the screen on which you are viewing it, but you can clearly discern far more than seven shades.
A scientifically precise device would show you that there is no limit; that the rainbow represents a continuous flow from red to violet, and is no more divided into a finite set of colors than the numerical range from 1 to 2 can be divided into a finite set of numbers. There are an infinite number of colors in the rainbow, because what determines color (as our eyes perceive it and our brains reconstruct it) is the wave’s frequency, the number of its cycles per second — and that number that can be varied smoothly across a range, just like any other number.
