The Electromagnetic Spectrum


Learning Objectives


The Electromagnetic Spectrum

Electromagnetic (EM) radiation exists in a large range of frequencies and wavelengths as you can see in the figure below (courtesy Prof. Julien Clinton, University of Wisconsin). This range is called the electromagnetic spectrum.

Diagram of EM spectrum

All electromagnetic radiation has characteristics of waves and particles (photons) . The wave properties are more apparent at lower frequencies while the "rays" of higher frequencies have more obvious particle behaviors. We will focus on the radio waves. (Note that the visible part of the spectrum is very small).

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.

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.

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

For this training, we will focus the radio waves, including microwave. This area is used by the military for communication, detection, weapons systems operation and other uses.

The image above 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

Electromagnetic radiation is is not the only type of radiation. Atomic particles such as protons, electrons (cathode or beta rays) and helium nuclei (alpha rays) are also called radiation. However, we will focus here only on photons or EM radiation.



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.

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