Red light—that is, light that our human eyes perceive as red—has a frequency of about 420 THz (terahertz). That means electric fields and magnetic fields are flipping back and forth about 420 trillion times each second. If your car wheels revolved that quickly, you could drive from one end of our solar system to the other in the time it takes to blink an eye. At that rate, each of those red-colored light waves is about 700 nm (700 billionths of a meter) long–about a tenth the size of a red blood cell–even smaller than a typical microscopic bacterium. As you shorten the wavelength—that is, increase the frequency, so you get more waves per second—red becomes yellow, then green, then blue, and then, at about twice the frequency of red, purple. Speed up the frequency even more and the light changes, moving beyond what we can see into the ultraviolet (UV) range, then X-rays, then gamma rays.