A picture that seems to show up a lot in high school physics is one like this: [PLAIN]http://www.kollewin.com/EX/09-15-03/electromagnetic-spectrum.jpg [Broken] There's the wide range of electromagnetic waves at different frequencies, of which we can only see a narrow band. My question is, why do red and purple seem to overlap so nicely? Why, for example, can we make things like below, where we go through the rainbow from red to purple and then back to red again? [URL]http://www.color-wheel-pro.com/pics/color-wheel1.jpg[/URL] From looking at the electromagnetic wave spectrum, it seems like there's no reason for purple to blend nicely with red at all, and yet it seems to in practice. Is there a good reason for this? What would happen with animals that can see different parts of the spectrum than us (like ultraviolet, for example)?