You would have to count pretty quickly to do this for light, however. Light waves vibrate or oscillate at the rate of some 400 million million times a second. That’s the vibration rate of waves of red light; violet results from vibrations that are just about twice that fast. With frequencies of this magnitude, discussion and handling of data and dimensions are cumbersome and rather awkward. Fortunately there is another approach. Let’s look again at our ocean waves. We see that there is a regularity about them (before they begin to break on the shore). The distance from one crest to the next is significant and is called the _wavelength_. Water waves are measured in feet, and in comparable units light waves are recorded in ten-millionths of an inch—again a very cumbersome number. Scientists therefore use the metric system[2] and have standardized a unit called the angstrom[3], which is equal to the one-hundred-millionth part of a centimeter (10⁻⁸ cm). Thus we find, as shown in Figure 3, that the visible light range runs from the violet at about 4000 angstroms to red at about 7000 angstroms. [Illustration: Figure 3 _The visible light spectrum ranges between approximately 4000 and 7000 angstroms._] Wavelength (Angstroms) Violet 4000-4300 Blue 4300-5000 Green 5000-5600 Yellow 5600-5800 Orange 5800-6100 Red 6100-7000 At roughly the same time that the wavelength of light was being determined, the German-British astronomer William Herschel performed an interesting experiment. He held a thermometer in turn in the various colors of light that had been spread out by an optical prism. As he moved the thermometer from the violet to the red, the temperature reading rose—and it continued to rise as he moved the instrument _beyond_ the red area, where no prismatic light could be seen. Thus Herschel discovered infrared rays (the kind of heat we get from the sun) adjoining the visible red light, and at the same time found that they were merely a continuation of the visible spectrum. Shortly thereafter, ultraviolet rays were found on the other end of the visible light band.