Electromagnetic waves

The previous page showed the generation of a pulse of electromagnetic radiation from an isolated electric charge. In reality, most objects have equal amounts of positive and negative charge, though small separations of these charges will produce electric dipole fields like the one pictured below:

The balance of charges cancels out the leading 1/𝑟2 component of the static field, leaving a dipole field that scales as 𝑠/𝑟3, where 𝑠 is the charge separation. However, changes in the charge configurations still produce propagating transverse fields that scale as 𝑎/𝑐2𝑟, as before. In particular, if the charges accelerate in a cyclical or continuous manner, the resulting radiative field is a stream of electromagnetic waves, as shown below.

Electromagnetic waves have a characteristic wavelength λ that is 𝑐 times the characteristic frequency 𝑓 of the underlying charge motion. The strength of the wave therefore scales as:

The familiar forms of electromagnetic radiation we detect with instruments have wavelengths ranging from metres (radio waves) down to femtometres (gamma rays). A narrow span of wavelengths, from about 400 to 700 nanometres, can be sensed by the human eye as visible light.

Source: Wikimedia

• Long wavelengths (microwaves and radio waves) are produced by non-thermal large-scale coherent electromagnetic processes, such as electric circuitry, or from diffuse plasmas accelerated by magnetic fields. The Earth's ionosphere blocks radio waves longer than about 10 m, while interstellar plasma blocks waves longer than about 10 km, which sets the long-wavelength limit of radio astronomy.
• Mid wavelengths (infrared through ultaviolet) are mostly produced by atomic energy transitions, and by thermal radiation from "ordinary" (atomic) matter. The lowest temperatures normally found in nature are around 3 kelvins, emitting thermal radiation at wavelengths around 1 mm: this usually taken to be long-wavelength end of the infrared band. A photon wavelength of around 10 nm or less is sufficiently energetic to break most molecular bonds and ionize atoms, and sets the short end of the ultraviolet band.
• Short wavevelengths (X-rays, gamma rays) are produced by nuclear energy transitions, or thermal radiation from matter at nuclear energies, or by high-energy nonthermal particle acceleration (e.g. from particles or dense plasmas boosted by strong electromagnetic fields). Nuclei start to dissociate under radiation in the sub-picometre range, and individual nucleons have rest energies equivalent to femtometre-wavelength photons, which broadly sets the upper end of photon energies that typically arise from matter in the present-day Universe.

Fundamentally, electromagnetic waves can be as short as a few attometres (10−18 m), below which electromagnetism per se no longer exists (it is replaced by a field combining electromagnetic and weak nuclear forces). In principle electromagnetic waves can be as long as the size of the Universe (a few ×1026 m), but will not propagate very far through the interstellar medium – furthermore, propagating wave-like behaviour only appears at distances more than a wavelength from the source (closer in, the 1/𝑟3 near-zone field dominates over the 1/𝑟 radiative field).