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 1. 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.
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The spectrum spans 19 orders of magnitude. It would take 10,000,000,000,000,000,000 of the shortest
gamma-ray wavelengths lined up end-to-end to stretch the same length as the longest radio wave.
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Electromagnetic spectrum
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 1/25th of an
inch in length.
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Electromagnetic spectrum
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 1/25,000th 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.
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