A Radio Wave is not a Gamma-Ray, a Microwave is not an X-ray... or is it?

We may think that radio waves are completely different physical objects or events than gamma-rays. They are produced in very different ways, and we detect them in different ways. But are they really different things? The answer is 'no'. Radio waves, visible radiation, X-rays, and all the other parts of the electromagnetic spectrum are fundamentally the same thing. They are all electromagnetic radiation.

Electromagnetic radiation can be described in terms of a stream of massless particles, each traveling in a wave-like pattern and moving at the speed of light. Each massless particle contains a certain amount (or bundle) of energy. Each bundle of energy is called a photon, and all electromagnetic radiation consists of these photons. The only difference between the various types of electromagnetic radiation is the amount of energy found in the photons. Radio waves have photons with low energies, microwaves have a little more energy than radio waves, infrared has still more, then visible, ultraviolet, X-rays, and... the most energetic of all... the gamma-rays.

Actually, the electromagnetic spectrum can be expressed in terms of energy, wavelength, or frequency. Each way of thinking about the EM spectrum is related to the others in a precise mathematical way. So why have three ways of describing things arisen, each with a different sets of physical units? After all, frequency is measured in cycles per second (which is called a Hertz), wavelength is measured in meters, and energy is measured in electron volts.

The answer is that scientists don't like to use big numbers when they don't have to. It is much easier to say or write "two kilometers or 2 km" than "two thousand meters or 2,000 m". So generally, scientists use whatever units are easiest for whatever they are working with. In radio astronomy, astronomers tend to use wavelengths or frequencies. This is because most of the radio part of the EM spectrum falls in the range from a about 1 cm to 1 km, and 1 kilohertz (kHz) to 1 megahertz (MHz). The radio is a very broad part of the EM spectrum. Infrared astronomers also use wavelength to describe their part of the EM spectrum. They tend to use microns (or millionths of meters) for wavelengths, so that they can say their part of the EM spectrum falls in the range 1 to 100 microns. Optical astronomers use wavelengths as well. In the older "CGS" version of the metric system, the units used were Angstroms. An Angstrom is equal to 0.0000000001 meters (10^-10 m in scientific notation)! In the newer "SI" version of the metric system, we think of visible light in units of nanometers or 0.000000001 meters (10^-9 m). Then we have the violet, blue, green, yellow, orange, and red light we know so well having wavelengths between 400 and 700 nanometers. This is a very tiny part of the EM spectrum. By the time you get to the ultraviolet, X-ray, and gamma-ray regions of the EM spectrum, lengths have become too tiny to think about any more. So scientists usually refer to these photons by their energies, which are measured in the tiny unit of electron volts. Ultraviolet radiation falls in the range from a few electron volts (eV) to a about 100 eV. X-ray photons have energies in the range 100 eV to 100,000 eV (or 100 keV). Gamma-rays then are all the photons with energies greater than 100 keV.