The electromagnetic spectrum is separated into categories of light according to wavelength (the distance between successive wave crests) or frequency (wave cycles per second). Radio waves occupy the long-wavelength end of the electromagnetic spectrum; radio waves have wavelengths that are greater than 1 mm (and can be meters long) and frequencies that range from a few hertz (Hz) to about 300 gigahertz (300 billion Hz). At the other end of the spectrum, there are gamma rays, which have very short wavelengths (less than the size of an atom) and frequencies above 10 exahertz (10¹⁹ Hz).

The wavelength, frequency, and energy of an elecromagnetic wave are mathematically related. According to the wave-model of light, f = v/λ (frequency = speed of light/wavelength).

In a vacuum, all electromagnetic radiation travels at approximately 300,000 km/s, a constant known as c, the speed of light in a vacuum. But the optical density of matter affects the speed at which light travels through it; the more optically dense a material is, the slower light will travel through it. For example, light travels more slowly through air than it does in a vacuum, and it travels even more slowly through water. As a result, when light travels from one medium to another, its speed changes. However, the frequency does not change as light travels through different materials, which means that wavelength also changes when there is a change in speed.

Where does electromagnetic radiation come from? The acceleration of charged particles, such as protons or electrons, generates electromagnetic radiation. For example, we can generate radio waves for broadcast by moving electrons along an antenna. In nature, there are two ways to produce electromagnetic radiation: thermal and nonthermal.

Thermal emission is the most well-known form of electromagnetic emission. All forms of matter emit electromagnetic radiation this way—their atoms and molecules are constantly moving around and bumping into each other. This temperature-dependent motion of charged particles emits light known as blackbody radiation; matter at lower temperatures emits radiation at longer wavelengths, and matter at higher temperatures emits radiation at shorter wavelengths. Objects that emit at radio wavelengths have a temperature less than 10 K; for example, the remnants of the Big Bang explosion, present throughout the universe at about 3 K, can be observed at radio wavelengths.

Other types of thermal emission include "free-free" emission (caused by the acceleration of free electrons in an ionized gas or plasma) and spectral line emission (caused by the transition of an electron in an atom from a higher to a lower energy level). The most abundant element in the universe, hydrogen, produces a spectral line in the radio region at a wavelength of 21 cm.

Sources of nonthermal emissions include synchrotron emission (the acceleration of charged particles in a magnetic field) and masers (microwave amplification by stimulated emission of radiation).

Astronomers look at radio emissions to study star-forming regions, active galactic nuclei, supernova remnants, and quasars.

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