Electromagnetic radiation is defined as the emission and transmission of energy in the form of electromagnetic waves (Chang, pp. 246). An electromagnetic wave consists of two waves at perpendicular planes, on representing the electrical field while the other represents the magnetic field. This spectrum is split up into about 81 octaves, much like the octaves of sound, which scientists have categorized in order to depict the usefulness of the different properties. Theoretically electromagnetic waves can be as long as the expansive width of the universe in one wave, to a wavelength the size of a quanta (the smallest amount of energy that can be transmitted). That, however, is at this point in time negligible because we either cannot detect wavelengths so extremely large or infinitesimally small. So generally the spectrum is regarded to be set between 30,000,000 km and .0001 angstroms (1 km is equal to 1 X 10^13 angstroms).
The longest and least energetic of the accepted wavelengths are radio waves. Their wavelengths range from 300,000 km to 30 cm and a frequency of 30 kHz to 3,000,000 kHz. The waves are used in the transmission of radio and television signals. AM and FM radio signals fall in the long-wave radio range, while TV signals use the short-wave radio range, somewhere in the 3 m wavelength. Radio waves take up about 30 of the 81 octaves.
The next slightly more energetic and shorter wavelength is microwaves. Obviously, most are familiar with microwaves since they are always being used to cook food, but microwaves are also used in radiotelescopes and radar, even the radiation from a neutral hydrogen atom falls in this range. Microwave range extends from wavelengths of 30 cm to .3 cm and frequencies of 1-100 billion cycles per second. This corresponds to 6 1/2 octaves.
Then comes the infrared range. While much of the radio spectrum passes easily through our atmosphere, infrared is almost completely blocked off due to absorption by gaseous molecules that reside there. Infrared waves are present during the transmission of heat. Its wavelengths range from .3 cm to .000076 cm with frequencies from 100 billion (10^11) to almost 10^14 cycles per second. 12 octaves are part of the infrared spectrum.
The visible spectrum with its single octave is the part of the spectrum that only the blind are denied. The atmosphere is highly transparent to the visible spectrum, and is one reason why biologists believe it is the only part of the spectrum our eyes are sensitive to. Our sun produces intense amounts of visible light, which also explains why we dont perceive the much less abundant radio wavelengths that are also impervious to our atmosphere. The visible wavelengths range from .76 microns (7600 angstroms) to .38 microns (3800 angstroms) with frequencies of slightly less than a quadrillion (10^15).
Many recognize UV or ultraviolet radiation because of modern societies obsession with tanning, and the cancerous growths that may follow. UV radiation is very energetic compared to the previous categories discussed and will burn with the prolonged exposure, which tanning is a mild form of. Not much UV can get through the ozone layer, but a little does, and our polluting industrial revolution has caused our protective layer to break down allowing more UV radiation in and larger health problems to the world. It has wavelengths from 3800-100 angstroms, with frequencies from a quadrillion (10^15) to slightly less than 100 quadrillion (10^17). UV occupies about 5 octaves.
X-rays, once again, are also well known to the public through their modern medical usage. They are very energetic in nature and extremely harmful to the living bodies, especially in the reproductive sites. They can easily pass through skin and reveal the underlying structure of animals. X-rays have been known to also cause cancer and even mutations. Prolonged exposure leads to radiation poisoning and certain death. Its wavelengths range from 100-.1 angstroms with frequencies of just under 10^17 to 10^20 cycles per second. It contains 10 octaves.
Gamma rays are the newly discovered form of radiation that is associated with space, and is given off in reactions concerning the nuclei of atoms. These are the most energetic and shortest wavelengths that are very, very lethal. They are often called cosmic rays and so far have no practical use other than extraterrestrial observation. Its frequencies are 100 quintillion (10^20) and higher with wavelengths than 1/10th of an angstrom.
Although scientists have established these arbitrary borders for the electromagnetic spectrum, there is truly no difference between them. Their only differences lie in the amount of energy they contain, their wavelengths, and their frequencies. They all behave as waves and particles, and all travel at the unerring speed of light, 300,000 Km per second.
Field, George B. and Chaisson, Eric J, The Invisible Universe: Probing the Frontiers of Astrophysics. Boston, Birkhauser, 1985.
Asimov, Isaac, Adding a Dimension. New York, Double Day, 1964.