The idea of electromagnetic radiation was first set out by Scottish Physicist James Clark Maxwell in 1865. he expanded on work done by Michael Faraday At the time no-one had a clue that they would lead to so many discoveries, including the radio filled world we live in now. In fact, for the next few years, although several people worked on Maxwell's equations, and Heinrich Hertz famously proved the theory correct, no one had any idea that there were practical uses for electromagnetic waves.
Coming from right down at the bottom of the electromagnetic spectrum, radio waves can be identified as having a frequency lower than 300 GHz and/or a wavelength longer than one millimeter, but within the radio spectrum there are many bands of wavelengths which are used for different purposes. The very lowest band however, with wavelengths longer than 100,000km, is unused - radiation of this type is natural and originates in the earth, the atmosphere, the sun and the other stars. Above that, the Extremely Low Frequency (ELF) and Super Low Frequency (SLF) ranges are reserved for use in communication with submarines deep below the sea.
Very Low Frequency (VLF)radio waves have a frequency between 3KHz and 30 KHz. They are somewhat easier to handle than ELF, but can only penetrate the sea to around 130 feet depending on the precise frequency used and the amount of salt in the water. They are used to communicate with submarines and for communication inside mines, avalanche beacons and wireless heart rate monitors, the type used in fitness training. VLF is also used in radio navigation beacons and for time signals.
The range below 9 KHz has not been allocated by the ITU (International Telecommunication Union) and in some countries it can be used without a license. Whistlers, very low frequency waves (usually) generated by lightning are often found in this range, most between 3 and 5 kHz. It was this type of radio signature that the spaceships Voyagers 1 and 2 detected around Jupiter, leading scientists to conclude that there was lightning on the giant planet. Hiss is yet another type of ELF/VLF wave in this case generated by the plasma in the Earth's magnetosphere or ionosphere.
Band five, the low frequency band is used for navigation, AM broadcasts (long wave) and time signals, while band six, the 300-3000kHz band is used for all medium wave broadcasts. Band seven is the region of the EM spectrum of interest to radio enthusiasts as this includes the amateur radio wavebands - wavelengths of 10 to 100 meters. 10 meters is typically used for amateur radio, while CB radio uses an 11 meter wavelength.
Beyond this range, in band 8, lies VHF with television and FM radio. Band 9 includes televisions, microwave ovens, mobile phones and wireless LANS as well as blue-tooth and GPS systems. Band 10 is described as Super High Frequency and devoted to microwaves, wireless networks, radar and wireless USB while the Extremely High Frequency range (EHF, 30 to 300 GHz) is used by radio astronomers and microwave radio relays. These radio waves have a wavelength between one centimeter and 1 millimeter and are not suitable for us over long distances. Rain, for example, can absorb the signal and reduce it's strength.
Beyond EHF is the highest part of the radio spectrum (but not the electro magentic spectrum) known as the Tera-hertz range as it falls between 300 Ghz and 3 Tera-hertz and has a wavelength between that of microwaves and far-infrared light. It has only recently become possible to generate radio waves in this Tera-hertz range at room temperature, previously super cooled equipment had been required. Scientists are still working on applications for the technology, however it seems likely that Tera-hertz waves (t-waves for short) will be able to penetrate both skin and fatty tissue and hence could be used as a possible replacement for x-rays which are far more harmful. They may also be used for imaging teeth. T-waves can also penetrate fabrics and plastic so may be used in security systems to scan people for concealed weapons.
CB occupies only a tiny portion of a tiny part of the vast spectrum of electromagnetic waves which range all the way from the huge wavelengths of radio to the tiny wavelengths of gamma rays which are smaller than the size of an atom. But don't let their tiny share of the EM spectrum fool you. Radio waves are easily able to change our lives.