Energy travels throughout the universe at the speed of light in the form of electromagnetic radiation. What that radiation is called depends on its energy level.
At the really high-energy end of the spectrum, you've got gamma rays. You're probably familiar with a close cousin to these: X-rays. They're the ones doctors and dentists use to probe for unusual structures inside your body. Radio waves fall at the extreme other end. Those radio waves are the ones that deliver music and news broadcasts to your home radios.
Ultraviolet rays, visible light, infrared radiation, and microwaves fall at energy levels in between.
Together, all of these types of radiation make up one long, continuous electromagnetic spectrum. Its energy travels in what's usually referred to as waves.
What separates one type of electromagnetic radiation from another is its wavelength. That's the length of a wave of that type of radiation. To identify the length of a wave of water in the sea, you would measure the distance from the crest (upper part) of one wave to the crest of another. Or you could measure from one trough (bottom part of a wave) to another.
It's more difficult to do, but scientists measure electromagnetic waves the same way—from crest to crest or from trough to trough. In fact, each segment of the energy spectrum is defined by this wavelength. Even what we refer to as the heat given off by radiators is a type of radiation—one that has wavelengths in the infrared portion of the spectrum.
Sometimes these segments of the electromagnetic spectrum are also described in terms of frequency. A radiation's frequency will be the inverse of its wavelength. So the shorter the wavelength, the higher its frequency. That frequency is typically measured in hertz, a unit which stands for cycles per second.
Stories that reference this explainer: