Spectrum: rainbows versus frequencies for sale

To most people, a spectrum is the visible rainbow: red light at one end, violet at the other, with all the colors in between forming a continuous band. Isaac Newton called it that because white light, when refracted through a prism, spreads into its component wavelengths. The word comes from the Latin for ghost or apparition, because the colored bands seem to appear and vanish.

In electrical engineering, spectrum means something narrower and more valuable: a set of radio frequencies allocated by government regulators for specific purposes. The Federal Communications Commission in the United States, for example, assigns spectrum to cellular networks, broadcast television, WiFi, aircraft communication and dozens of other uses. A given spectrum assignment might be 850 to 900 megahertz for one provider, or 2.4 to 2.5 gigahertz for another. These ranges are not continuous in practice; they are divided into channels with gaps between them to prevent interference.

The divergence happened because both senses describe the same physical reality: electromagnetic waves arranged by frequency. The visible light spectrum is the tiny slice of all electromagnetic waves that human eyes can detect, roughly 400 to 700 nanometers in wavelength. Radio frequencies are much lower energy and longer wavelength, measured in meters or fractions of a meter. When engineers began using radio in the early twentieth century, they borrowed the word spectrum to describe which frequencies they were permitted to transmit on.

What makes spectrum scarce and valuable in engineering is not the physics, but the regulation. The electromagnetic spectrum is infinite, but usable radio bands are finite, and many devices want to use the same frequencies. Regulators must prevent interference by assigning non-overlapping spectrum to different services. A cellular company might pay billions at auction for the right to use a specific block of spectrum in a specific country. That economic meaning has almost no parallel in physics or optics, where spectrum is simply the way light naturally sorts itself.