Electromagnetic radiation
Electromagnetic radiation is the propagation through space of a wave in an electromagnetic field, which carries energy from one place to another. When any wire (or other conducting object such as an antenna) conducts alternating current electromagnetic radiation is propagated at the same frequency as the electric current. Depending on the circumstances, it may behave as waves or as particles. As a wave, it is characterized by a velocity (the velocity of light), wavelength, and frequency. Electromagnetic radiation with wavelengths between about 400nm and 800nm (in the air), is known as visible light. Other wavelengths of electromagnetic radiation are known by various names, including radio, microwaves, X-rays, and gamma rays. When considered as particles, they are known as photons, and each has an energy related to the frequency of the wave given by Planck's relation E = h f, where E is the energy of the photon, h is Plancks constant and f is the frequency of the wave.
Generally Electromagnetic radiation is classified by wavelength along the electromagnetic spectrum as follows:
| Wavelength | Frequency | Energy | |
|---|---|---|---|
| Gamma rays | < 10 pm | >30.0 EHz | >19.9E-15 J |
| X-rays | < 10 nm | >30.0 PHz | >19.9E-18 J |
| Extreme UV | < 200 nm | >1.5 PHz | >993E-21 J |
| Near UV | < 380 nm | >789 THz | >523E-21 J |
| Visible | < 780 nm | >384 THz | >255E-21 J |
| Near IR | < 2.5 um | >120 THz | >79.5E-21 J |
| Mid IR | < 50 um | >6.00 THz | >3.98E-21 J |
| Far IR | < 1 mm | >300 GHz | >199E-24J |
| Microwaves | < 100mm | >3.0 GHz | >1.99e-24J |
| Ultrahigh Frequency Radio | <1m | >300 MHz | >1.99e-25J |
| Very High Frequency Radio | <10m | >30 MHz | >2.05e-26J |
| Shortwave Radio | <180m | >1.7 MHz | >1.13e-27J |
| Medium Wave (AM) Radio | <650m | >650 KHz | >4.31e-28J |
| Longwave Radio | <10Km | >30 KHz | >1.98e-29J |
| Very Low Frequency Radio | >10Km | <30KHz | <1.99e-29J |
See SI prefix
Note: between 300 GHz and the mid-infrared, the absorption of electromagnetic radiation by Earth's atmosphere is so great that the atmosphere is effectively opaque to electromagnetic radiation, until the atmosphere becomes transparent again in the so-called infrared and optical window freqency ranges.
While the above classification scheme is generally accurate, in reality there is often some overlap between neighboring types of electromagnetic radiation. For example some low energy gamma-rays actually have a longer wavelength than some high energy X-rays. This is possible because originally "gamma-ray" was the name given to the photons generated from nuclear decay, whereas X-rays on the other hand were generated by electronic transitions involving highly energetic inner electrons. Therefore the distinction between gamma-ray and x-ray is related to the radiation source rather than the radiation wavelength. Generally, nuclear transitions are much more energetic than electronic transitions, so most gamma-rays are more energetic than x-rays. However, there are a few low-energy nuclear transitions (eg. the 14.4 keV nuclear transition of Fe-57) that produce gamma-rays that are less energetic than some of the higher energy x-rays.