How do stars, galaxies, and other cosmic bodies relay information about their existence? They emit visible light, radio waves, x-rays, and microwaves, which hurtle across the cosmos at the speed of light—an unimaginably fast 671,000,000 mph! These electromagnetic waves are the fundamental source of information about the universe. Even apart from their speed, their ability to reach us at all was once a deep and profound mystery to physicists. Most waves, such as sound waves or ripples in a pond, require a medium (air, water, or a solid material) to travel through. Since the cosmos is mostly empty space, how can waves travel through it?

This cosmic puzzle was solved by physicists conducting earthbound experiments with electricity and magnetism. As shown in Fig. 1, an electromagnetic (EM) wave consists of both electric and magnetic fields, “inseparably coupled and mutually sustaining . . . regenerating each other in an endless cycle,” requiring no medium to travel in and no source to keep it going (Hecht, Ch. 3, p. 54). EM waves come in a vast continuous spectrum of wavelengths, as shown in Fig. 2. Radio waves, which transmit music and television through our atmosphere, have long wavelengths, oscillating slowly up and down. X-rays, used to probe inside our bodies for tooth cavities and broken bones, are among the shortest wavelengths. Infrared light, visible light—the familiar RedOrangeYellowGreenBlueIndigoViolet (ROYGBIV) spectrum of rainbows and glass prisms—and ultraviolet light all have intermediate wavelengths. As the wavelength decreases across the EM spectrum, the energy of the waves increases: gamma rays are extremely energetic, while radio waves are not.

Unaided, the human eye can only see a tiny slice of the electromagnetic spectrum—ROYGBIV, referred to as the visible spectrum—but the entire EM spectrum is available for view, provided we can find the right kind of “eye.” You are probably familiar with night vision goggles, which allow us to “see” infrared waves that are related to the temperature of an object. Scientists have developed detectors for every type of EM wave, and when we use different detectors, ordinary objects can look remarkably strange. We can also “see” magnificent images of celestial objects in every wavelength range—low-energy radio waves, visible light, ultraviolet light, and finally x-rays and gamma rays. In Fig. 3, we show these images for the Crab Nebula across the electromagnetic spectrum.  Each of these gives us different information about the shape, size, and material of the nebula.

We can also learn about objects by bombarding them with electromagnetic waves and using our detectors to see what bounces back. The vast range of EM wavelengths allows us to choose a wavelength to match the scale of the object we want to study. X-rays are particularly powerful for visualizing the atomic structure of materials, since their wavelengths are similar in size to a typical atom (see Dude, Where My Atoms At?). On the other end of the spectrum, radar—which is actually an acronym for radio detection and ranging—uses radio waves to locate and track larger objects, like tornadoes. And you can use the visible light from laser pointers to measure the thickness of a strand of your own hair! Fig. 4 shows how different EM wavelengths compare in size to other familiar objects.

In Fig. 5, you’ll find a cornucopia of examples of electromagnetic waves you encounter—and rely on—in your everyday life. What others can you think of?

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