# 24.3 The electromagnetic spectrum

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## 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] .

Electromagnetic waves
Type of EM wave Production Applications Life sciences aspect Issues
Radio&TV Accelerating charges Communications Remote controls MRI Requires controls for band use
Microwaves Accelerating charges&thermal agitation Communications Ovens Radar Deep heating Cell phone use
Infrared Thermal agitations&electronic transitions Thermal imaging Heating Absorbed by atmosphere Greenhouse effect
Visible light Thermal agitations&electronic transitions All pervasive Photosynthesis Human vision
Ultraviolet Thermal agitations&electronic transitions Sterilization Cancer control Vitamin D production Ozone depletion Cancer causing
X-rays Inner electronic transitions and fast collisions Medical Security Medical diagnosis Cancer therapy Cancer causing
Gamma rays Nuclear decay Nuclear medicineSecurity Medical diagnosis Cancer therapy Cancer causing Radiation damage

## Connections: waves

There are many types of waves, such as water waves and even earthquakes. Among the many shared attributes of waves are propagation speed, frequency, and wavelength. These are always related by the expression ${v}_{\text{W}}=\mathrm{f\lambda }$ . This module concentrates on EM waves, but other modules contain examples of all of these characteristics for sound waves and submicroscopic particles.

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,

$c=\mathrm{f\lambda }.$

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.

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