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.—Janet Raloff