Electromagnetic waves can be described by their wavelengths, energy, and frequency. All three of these things describe a different property of light, yet they are related to each other mathematically. This means that it is correct to talk about the energy of an X-ray or the wavelength of a microwave or the frequency of a radio wave.

In fact, X-rays and gamma-rays are usually described in terms of energy, optical and infrared light in terms of wavelength, and radio in terms of frequency. This is a scientific convention that allows the use of the units that are the most convenient for describing whatever energy of light you are looking at. After all - there is a huge difference in energy between radio waves and gamma-rays. Here's an example. Electron-volts, or eV, are a unit of energy often used to describe light in astronomy. A radio wave can have an energy of around 4 x 10^-10 eV - a gamma-ray can have an energy of 4 x 10⁹ eV. That's an energy difference of 10¹⁹ (or ten million trillion) eV!

We already know that when we talk about wavelength, we are talking about the distance between two peaks of a wave. Wavelength is usually measured in meters (m). Frequency is the number of cycles of a wave to pass some point in a second. The units of frequency are thus cycles per second, or Hertz (Hz). Radio stations have frequencies. They are usually equal to the station number times 1,000,000 Hz. For instance, - the local Washington, DC station HFS has a frequency of 99.1 million Hz in the FM radio band.

Do you know what a centimeter (cm) is? Or a kilogram (kg)? "Centi" and "kilo" are prefixes used to designate a certain number. "Centi" is one hundreth, and "kilo" is one thousand, so a centimeter is one hundreth of a meter. A kilogram is 1000 grams. In science, it is inconvenient to talk about very large or very small numbers so scientists abbreviate them. "Nano", (or "n" for short) means one billionth or 10^-9. Wavelengths of optical light have units of nanometers (nm). One nm is equal to one billionth of a meter. "Micro" (abbreviated by the Greek letter mu or µ) is one millionth or 10^-6. A micrometer, or one thousandth of a meter is sometimes called a "micron". "Milli" (m) is one thousandth or 10^-3. "Centi" (c) is one hundreth or 10^-2. "Kilo" is one thousand or 10³. "Mega" (M) is one million or 10⁶. The frequency of HFS is 99.1 MHz - that is 99.1 Megahertz. One billion is "giga" (G) or 10⁹.

We already know that light acts like a wave, but did you know that sometimes it acts like a particle? We call particles of light photons. Low-energy photons, like radio photons, tend to behave more like waves, while higher energy photons (i.e. X-rays) behave more like particles. That's another reason that we don't talk about X-ray waves very often. Instead we talk about individual X-rays and their energies.