ELECTROMAGNETIC SPECTRUM

By David F. Salisbury
Oct. 9, 2001

The electromagnetic spectrum consists of the entire range of radiation that is made of oscillating electrical and magnetic fields. The visible rainbow produced by a prism is just a sliver of the entire spectrum. It extends from radio waves, which can be thousands of feet long, to gamma rays with wavelengths shorter than the diameter of the nucleus of an atom . You can think of light as pure energy. It doesn't have any mass and it travels through space as an electromagnetic wave train. Each color, or wavelength, is associated with a specific frequency and amount of energy. The longer the wavelength, the slower the frequency and the less energy a wave carries. In many cases electromagnetic radiation acts like a classical wave, such as spreading out after passing through a pinhole. However, light in its various forms is also made up of discrete particles, called photons and in many circumstances acts like a stream of particles.

Radio waves are what AM/FM radio, wireless telephones, and television stations use to send you popular songs, carry your conversations and allow you to watch your favorite sitcom. Radio waves are produced by causing electrons to vibrate back and forth in a metal antenna. They are the longest waves in the electromagnetic spectrum and can vary from more than 100,000 feet down to an inch.

Microwave radiation is used for radar, transmitting large amounts of information across the country and warming food and popping popcorn. Microwaves range from an inch to 1/25^th of an inch in length.

Infrared radiation is associated with heat, although even objects like ice, that we consider cold, emit IR waves. Radiant heaters use infrared light to directly warm objects without heating the air around them. IR sensors are also used in home security systems to detect intruders. Vanderbilt's free-electron laser operates in this region of the spectrum. Infrared waves range from 1/25^th of an inch down to a few millionths of an inch. The standard unit of measurement in this region is the micron, which is one millionth of a meter, or 1/25,000^th of an inch.

Visible light matters most to people because it is what our eyes can see. But it is a tiny sliver of the entire electromagnetic spectrum. We can only see wavelengths ranging between 0.4 to 0.7 microns. If you were building a scale model of the spectrum and set the width of visible light equal to the width of a human hair, then the entire spectrum would stretch about a light year, one quarter the distance to the nearest star!

Ultraviolet or UV radiation is produced by the Sun along with visible light. With wavelengths between a tenth to a thousandth of a micron, UV carries a greater punch than visible light. That is the reason it causes sunburn and skin cancer. Fortunately, the atmosphere, including the ozone layer, filters out much of the ultraviolet radiation before it reaches the surface.

X-rays range from a thousandth to a 100 thousandth of a micron or, in smaller units, from a nanometer to a hundredth of a nanometer. The smallest X-ray wavelengths are comparable to the size of an atom. X-rays. The first medical X-ray in America was taken at Dartmouth College in 1896. The monochromatic X-ray developed at Vanderbilt holds promise for significantly improving the quality and safety of medical X-rays.

Gamma-rays are even more energetic than X-rays. The only natural sources of gamma rays on Earth are radioactive materials. However, they are created artificially by high-energy particle accelerators and nuclear power plants. Gamma-ray wavelengths vary from a hundredth of a nanometer down to a few hundred thousandths of a nanometer-lengths comparable to those found in the nucleus of an atom. Gamma-rays are produced by the some of the most energetic events in the Universe, such as stellar explosions and matter falling into massive black holes.