Reality → Energy → Light → Spectrum
| μm | THz | eV | |
|---|---|---|---|
| ↑ | |||
| AM radio | |||
| 105 | 3×10-3 | 10-5 | |
| 1 | 300 | 1 | |
| 10-5 | 3×107 | 105 | |
| ↓ |
The spectrum of electromagnetic radiation can be expressed in wavelength, frequency, or photon energy [1] . It spans an enormous bandwidth of more than 20 orders of magnitude [2] , from long-wave, low-frequency, and low-energy radio waves to extremely short-wave, high-frequency, and high-energy gamma rays. Radio, microwave, heat, light, UV, X-ray, and ionizing radiation from nuclear reactions, despite their different sources and effects, are all manifestations of electromagnetic radiation, differentiated basically by the photon’s energy. AM radio wavelengths range from several kilometers down to 10 meters [3] . FM radio and TV use the band from 10 meters down to 1 decimeter, followed by microwaves which can be as short as 1 millimeter, and then by infrared radiation which spans the full micrometer (μm or micron) range (from 1,000 μm down to 0.8 μm) [4] . Then follows visible light, in a narrow band from about 800 nanometer (dark red) to 400 nanometer (violet), equivalent to a frequency of 400 to 800 terahertz, or a photon energy of 2 to 3 electronvolt (eV). Still shorter waves are normally distinguished by their photon energy. UV ranges from 3 eV to more than 1,000 eV, and X-rays from 1 keV to more than 100 keV [5] . Gamma rays extend from about 100 keV into the TeV range, or to billions of times the energy of light [6] .
Wavelength λ and frequency f are related by the simple formula λ = c / f , and energy and frequency by E = h f , where c is the speed of light and h the Planck constant.
The wavelength of known electromagnetic radiation occurring in nature ranges from hundredths of attometer (cosmic gamma rays at tens of TeV energy) to tens of thousand km (atmospheric Schuman resonances at 3 Hz frequency), i.e., over 24 orders of magnitudes (see Sheet). Theoretically, there are no limits, i.e., the longest wavelength might be as long as the universe and the shortest wavelength as short as the smallest subatomic unit.
Radio waves can also be much longer. Reportedly, extremely low frequency waves of less than 100 hertz (corresponding to a wavelength of several thousand km) have been tested for signal transmission to US and Russian submarines (the difficulty was to find suitable antennas).
Infrared is normally known as heat radiation (though the shorter infrared (near visible light) hardly transmits heat). The temperature effect is caused by atomic and molecular vibrations at frequencies of the long (far) infrared. All bodies emit and absorb thermal radiation, at any temperature.
UV radiation can be energetic enough to cause ionization, i.e., knock out electrons from atoms or molecules and thus damage living tissue.
There is no distinct border between high-energetic (hard) X-rays and low-energetic gamma rays. Gamma rays are the most penetrating component of radiation from nuclear processes. Gamma rays from uranium decay start at about 100 keV and extend into the MeV range. NASA’s Fermi Gamma-ray Space Telescope is equipped with sensors for up to 300 GeV.