MultiFazed: It doesn’t appear cyclic to the human eye. It appears cyclic to the human *brain*.
Our eyes can detect 3 “regions” of color: red, green, and blue. If we detect some combinations of those, we typically perceive that as an “in-between” color. For example, orange light stimulates both the red and green sensing cells in our eyes. So stimulating the red and green cells is what we perceive as “orange”. And, interestingly, if we just use red and green light (no orange light), we can stimulate those cells exactly the same as orange light, and so we still see orange. In fact, that’s the basis for how computer and phone displays work: They only emit red, green, and blue light, and our brains perceive combinations of those as other colors.
But here comes the strangeness! What happens when you stimulate the red and blue cells in the eye with red and blue light? Well, your first guess is that we should perceive the color that is “in between” red and blue on the spectrum. But that color is green, and we’re specifically *not* stimulating the green-detecting cells in our eyes. However, your brain isn’t really capable of seeing it as two different colors (red and blue) simultaneously, so it invents a new color! Purple!
That’s right, purple, the color that allows our sense of the spectrum to be cyclical, *isn’t a real color*. There is no such thing as a purple photon of light. Purple can *only* be perceived by the human brain as a side effect of the limitations of our visual system.