The Electromagnetic Spectrum: an Overview

In this module we examine how electromagnetic waves are classified into categories such as radio, infrared, ultraviolet, and so on, so that we can understand some of their similarities as well as some of their differences. We will also find that there are many connections with previously discussed topics, such as wavelength and resonance. A brief overview of the production and utilization of electromagnetic waves is found in Table 11.4.1. Note that atomic and/or molecular electron transitions—that is, electrons changing their energy levels within atoms or molecules—are among the production mechanisms for a majority of the types of electromagnetic waves listed in the table, from infrared through X-rays.

As noted before, an electromagnetic wave has a frequency and a wavelength associated with it and travels at the speed of light, or $c$. The relationship among these wave characteristics can be described by $v_{\text{W}}=f\lambda$, where $v_{\text{W}}$ is the propagation speed of the wave, $f$ is the frequency, and $\lambda$ is the wavelength. Here $v_{\text{W}}=c$, so that for all electromagnetic waves,

Thus, for all electromagnetic waves, the greater the frequency, the smaller the wavelength.

Gamma ray: a photon emitted in a nuclear transition; in astronomy, the term also includes very high-energy photons produced by other processes.

Figure 11.4.1 shows how the various types of electromagnetic waves are categorized according to their wavelengths and frequencies—that is, it shows the electromagnetic spectrum. Many of the characteristics of the various types of electromagnetic waves are related to their frequencies and wavelengths, as we shall see.