The Electromagnetic Spectrum

Learning Objectives

• Be familiar with the various types of EM radiation.
• Get a feeling for the wide variety of EM regions used by the military.
  

The Electromagnetic Spectrum

Electromagnetic (EM) radiation exists in a large range of frequencies and wavelengths (Figure 1). This range is called the electromagnetic spectrum.

Figure 1. Diagram of the electromagnetic spectrum. All electromagnetic radiation has characteristics of particles (photons) and waves. The wave properties are more apparent at lower frequencies while the "rays" of higher frequencies have more obvious particle behaviors. This class will focus on the visible and longer wavelengths. Notice that there is a one-to-one relationship between wavelength, frequency and energy of one photon. Higher frequencies correspond to shorter wavelengths and higher energy photons. ALS is the "Advanced Light Source" at the Berkeley Lab. This diagram is from a lecture series on radio communications by Prof. Julien Clinton "Clint" Sprott, University of Wisconsin.

Radio Waves and Microwaves
This type of radiation travels easily through the atmosphere and clouds. It is generated by accelerating electrons in antennas. Microwaves are usually considered to be a subset of radio waves. A majority of this class will be devoted to how the atmosphere affects these waves.

Infrared Radiation
This type of radiation is emitted by all matter that is at typical temperatures found on the earth's surface and atmosphere. Infrared radiation is absorbed by clouds and solid objects. Some infrared wavelengths travel long distances through the clear atmosphere while other wavelengths are absorbed in a few meters.

Visible Radiation
This is the main type of radiation that is emitted by the sun, other very hot objects such as incandescent light bulbs and from excited atoms in lasers and florescent light bulbs. It travels quite easily through the clear atmosphere. This is the only type of EM radiation that can be seen by people.

Ultraviolet Radiation
This is also emitted by the sun and other high energy (high temperature) sources. Most of it is absorbed in the upper atmosphere. It can travel a short distance into some solid objects such as skin, which is why it causes sun burns.

X-Rays
These are emitted by very high energy sources such as supernovas, radioactive material, and x-ray machines. They can travel some distance through many solid objects, making them useful for detection of contraband and for medical uses. They are virtually unaffected by the atmosphere.

Gamma Rays
These are emitted by extremely high energy sources such as nuclear explosions, and gases being sucked into black holes. They can travel great distances through solid objects and are very harmful to people. They are unaffected by the atmosphere.

Military Uses

	Click on the spectrum icon to the left to open a new window which shows some of the military and civilian uses of the radio part of the EM spectrum. The new window is large, when you have finished inspecting it you may want to minimize or delete.
Link to Figure 2

Figure 2 shows only a few of the uses and frequency locations employed for military uses. As you can see, the military uses many different locations within the EM spectrum. Notice that radio waves (which also include microwaves) have there own spectrum and are divided into several bands: ELF, ULF, VLF, LF, MF, HF, VHF, UHF, VHF and SHF. For now, you just need to know that these bands represent different radio frequency ranges. In later modules, we will discuss the characteristics and details of these different radio bands.

Other Types of Radiation

Summary

The EM spectrum encompasses a large range of wavelengths and frequencies. The military uses many different regions within the EM spectrum.

Study Questions

1. What is the approximate frequency of EM radiation with a wavelength of 1 meter?
2. What is the approximate frequency used for DMSP satellite downlinks? Hint: See figure 2.

Last Update: 1/12/2004