One of the most fascinating movements in science has been the constant expansion since then of both ends of the radiating-wave spectrum. The result has come to be called the _electromagnetic spectrum_, which, as we see in Figure 4, encompasses a wide variety of apparently different kinds of radiation. Above the visible band (the higher frequencies), we find ultraviolet light, X rays, gamma rays, and some cosmic rays; below it are infrared radiation, microwaves, and radio waves. Only a small proportion of the total spectrum is occupied by the visible band. Another point of interest is the inverse relationship between wavelength and frequency. As one goes up the other goes down.[4] [Illustration: Figure 4 _Visible light region spans a tiny portion of the total electromagnetic spectrum._] Frequency (cps) Wavelength Angstroms Cosmic rays 10²² 0.0001 0.001 10²⁰ Gamma rays 0.01 0.1 10¹⁸ X rays 1 10 10¹⁶ Ultraviolet radiation 100 1,000 Visible light 10¹⁴ 10,000 Infrared radiation 100,000 Angstroms 0.01 10¹² Millimeter waves 0.1 10¹⁰ Microwaves, radar 1 10 10⁸ TV and FM radio 100 Short wave 1,000 10⁶ AM radio 10,000 Low frequency communications 100,000 10,000 = 10⁴ 1,000,000 These many kinds of rays and waves vary tremendously in the ways they interact with matter. But they are all part of a single family. The only difference among them, as with the colors of the rainbow, lies in their wavelengths. In a few cases, as we shall see later, the mode of generation is also different.