Learning Objectives
By the end of this section, you will be able to do the following:
- 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 Table 7.1.
Type of EM Wave | Production | Applications | Life Sciences Aspect | Issues |
---|---|---|---|---|
Radio and 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 abnormal cell growth control | Vitamin D production | Ozone depletion cause abnormal cell growth |
X-rays | Inner electronic transitions and fast collisions | Medical Security | Medical diagnosis Cancer therapy | Cause abnormal cell growth |
Gamma rays | Nuclear decay | Nuclear medicineSecurity | Medical diagnosis Cancer therapy | Causes abnormal cell growth 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 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 . The relationship among these wave characteristics can be described by where is the propagation speed of the wave, is the frequency, and is the wavelength. Here so that for all electromagnetic waves,
Thus, for all electromagnetic waves, the greater the frequency, the smaller the wavelength.
Figure 7.10 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.
Electromagnetic Spectrum: Rules of Thumb
Three rules that apply to electromagnetic waves in general are as follows:
- High-frequency electromagnetic waves are more energetic and are more able to penetrate than low-frequency waves.
- High-frequency electromagnetic waves can carry more information per unit time than low-frequency waves.
- The shorter the wavelength of any electromagnetic wave probing a material, the smaller the detail it is possible to resolve.
Note that there are exceptions to these rules of thumb.