Energy travels throughout the universe at the speed of light as radiation. What that radiation is called depends on its energy level.
At the really high-energy end of the spectrum are gamma rays. They’re close cousins to the X-rays that doctors and dentists use to probe for unusual structures in your body. Radio waves fall at the extreme other end of the spectrum. They’re used to (among other things) 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 make up one long, continuous electromagnetic spectrum of light. Its energy travels in what are usually referred to as waves.
What separates one type of this radiation from another is its wavelength. That’s the length of a wave that makes up each type of radiation. To identify the length of a wave 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 — infrared rays.
Parts of the electromagnetic spectrum also can be described in terms of their 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.