24.3 The electromagnetic spectrum

Learning objectives

By the end of this section, you will be able to:

• List three “rules of thumb” that apply to the different frequencies along the electromagnetic spectrum.
• Explain why the higher the frequency, the shorter the wavelength of an electromagnetic wave.
• Draw a simplified electromagnetic spectrum, indicating the relative positions, frequencies, and spacing of the different types of radiation bands.
• List and explain the different methods by which electromagnetic waves are produced across the spectrum.

The information presented in this section supports the following AP® learning objectives and science practices:

• 6.F.1.1 The student is able to make qualitative comparisons of the wavelengths of types of electromagnetic radiation. (S.P. 6.4, 7.2)

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 [link] .

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 $v_{\text{W}}=\mathrm{f\lambda }$ , 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.

[link] 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.