Electromagnetic radiation is produced by accelerating charged particles such as from the Sun and other celestial bodies or artificially generated for various applications. Its interaction with matter depends on wavelength, influencing its uses in communication, medicine, industry, and scientific research. Radio waves enablebroadcasting andwireless communication, infrared is used inthermal imaging, visible light is essential for vision, and higher-energy radiation, such as X-rays and gamma rays, is applied in medical imaging, cancer treatment, and industrial inspection. Exposure to high-energy radiation can pose health risks, making shielding and regulation necessary in certain applications.
The relative wavelengths of the electromagnetic waves of three different colours oflight (blue, green, and red) with a distance scale in micrometers along the x-axis
Electromagnetic radiation is produced by accelerating charged particles and can be naturally emitted,[8][9] as from the Sun and other celestial bodies, or artificially generated for various applications. The energy in electromagnetic waves is sometimes calledradiant energy.[10][11] The electromagnetic waves' energy does not need a propagating medium to travel through space; they move through a vacuum at the speed of light.[12]
Electromagnetic waves can be imagined as a self-propagating transverse oscillating wave of electric and magnetic fields. This 3D animation shows a plane linearly polarized wave propagating from left to right. The electric and magnetic fields in such a wave are in phase with each other, reaching minima and maxima together.
Electric and magnetic fields obey the properties ofsuperposition. Thus, a field due to any particular particle or time-varying electric or magnetic field contributes to the fields present in the same space due to other causes. Further, as they arevector fields, all magnetic and electric field vectors add together according tovector addition.[13] For example, in optics two or more coherent light waves may interact and by constructive or destructiveinterference yield a resultant irradiance deviating from the sum of the component irradiances of the individual light waves.[14] The electromagnetic fields of light are not affected by traveling through static electric or magnetic fields in a linear medium such as a vacuum. However, in nonlinear media, such as somecrystals, interactions can occur between light and static electric and magnetic fields—these interactions include theFaraday effect and theKerr effect.[15][16]
Inrefraction, a wave crossing from one medium to another of differentdensity alters itsspeed and direction upon entering the new medium. The ratio of the refractive indices of the media determines the degree of refraction, and is summarized bySnell's law. Light of composite wavelengths (natural sunlight) disperses into a visiblespectrum passing through a prism, because of the wavelength-dependentrefractive index of theprism material (dispersion); that is, each component wave within the composite light is bent a different amount.[17]
EM radiation exhibits both wave properties andparticle properties at the same time (known aswave–particle duality). Both wave and particle characteristics have been confirmed in many experiments. Wave characteristics are more apparent when EM radiation is measured over relatively large timescales and over large distances while particle characteristics are more evident when measuring small timescales and distances. For example, when electromagnetic radiation is absorbed by matter, particle-like properties will be more obvious when the average number of photons in the cube of the relevant wavelength is much smaller than 1. It is not so difficult to experimentally observe non-uniform deposition of energy when light is absorbed, however this alone is not evidence of "particulate" behavior. Rather, it reflects the quantum nature ofmatter.[18] Aquantum theory of the interaction between electromagnetic radiation and matter such as electrons is described by the theory ofquantum electrodynamics.
Electromagnetic waves can bepolarized, reflected, refracted, ordiffracted, and can interfere with each other.[19][20][21] Some experiments display both the wave and particle natures of electromagnetic waves, such as the self-interference of a singlephoton.[22] When a low intensity light is sent through aninterferometer it will be detected by aphotomultiplier or other sensitive detector only along one arm of the device, consistent with particle properties, and yet the accumulated effect of many such detections will be interference consistent with wave properties.
Representation of the electric field vector of a wave of circularly polarized electromagnetic radiation
In homogeneous, isotropic media, electromagnetic radiation is atransverse wave,[23] meaning that its oscillations are perpendicular to the direction of energy transfer and travel. It comes from thefollowing equations:These equations predicate that any electromagnetic wave must be a transverse wave, where the electric fieldE and the magnetic fieldB are both perpendicular to the direction of wave propagation. The electric and magnetic parts of the field in an electromagnetic wave stand in a fixed ratio of strengths to satisfy the twoMaxwell's equations that specify how one is produced from the other. In dissipation-less (lossless) media, theseE andB fields are also in phase, with both reaching maxima and minima at the same points in space.
In thefar-field EM radiation which is described by the two source-free Maxwellcurl operator equations, a time-change in one type of field is proportional to the curl of the other. These derivatives require that theE andB fields in EMR are in phase. An important aspect of light's nature is itsfrequency. The frequency of a wave is its rate of oscillation and is measured inhertz, theSI unit of frequency, where one hertz is equal to one oscillation per second. Light usually has multiple frequencies that sum to form the resultant wave. Different frequencies undergo different angles of refraction, a phenomenon known asdispersion.
A monochromatic wave (a wave of a single frequency) consists of successive troughs and crests, and the distance between two adjacent crests or troughs is called thewavelength. Waves of the electromagnetic spectrum vary in size, from very long radio waves longer than a continent to very short gamma rays smaller than atom nuclei. Frequency is inversely proportional to wavelength, according to the equation:[24]
wherev is the speed of the wave (c in a vacuum or less in other media),f is the frequency, andλ is the wavelength. As waves cross boundaries between different media, their speeds change but their frequencies remain constant.
Electromagnetic waves in free space must be solutions of Maxwell'selectromagnetic wave equation. Two main classes of solutions are known, namely plane waves and spherical waves. The plane waves may be viewed as the limiting case of spherical waves at a very large (ideally infinite) distance from the source. Both types of waves can have a waveform which is an arbitrary time function (so long as it is sufficiently differentiable to conform to the wave equation). As with any time function, this can be decomposed by means ofFourier analysis into itsfrequency spectrum, or individual sinusoidal components, each of which contains a single frequency, amplitude, and phase. Such a component wave is said to bemonochromatic.
Interference is the superposition of two or more waves resulting in a new wave pattern. If the fields have components in the same direction, they constructively interfere, while opposite directions cause destructive interference. Additionally, multiple polarization signals can be combined (i.e. interfered) to form new states of polarization, which is known as parallel polarization state generation.[25]
In electromagnetic radiation (such as microwaves from an antenna, shown here) the termradiation applies only to the parts of theelectromagnetic field that radiate into infinite space and decrease in intensity by aninverse-square law of power, such that the total energy that crosses through an imaginary sphere surrounding the source is the same regardless of the size of the sphere. Electromagnetic radiation thus reaches thefar part of the electromagnetic field around a transmitter. A part of thenear field (close to the transmitter) includes the changingelectromagnetic field, but that is not electromagneticradiation.
Maxwell's equations established that some charges and currents (sources) produce localelectromagnetic fields near them that do not radiate. Currents directly produce magnetic fields, but such fields of amagnetic-dipole–type that dies out with distance from the current. In a similar manner, moving charges pushed apart in a conductor by a changing electrical potential (such as in an antenna) produce anelectric-dipole–type electrical field, but this also declines with distance. These fields make up thenear field. Neither of these behaviours is responsible for EM radiation. Instead, they only efficiently transfer energy to a receiver very close to the source, such as inside atransformer. The near field has strong effects on its source, with any energy withdrawn by a receiver causing increasedload (decreasedelectrical reactance) on the source. The near field does not propagate freely into space, carrying energy away without a distance limit, but rather oscillates, returning its energy to the transmitter if it is not absorbed by a receiver.[30]
By contrast, thefar field is composed ofradiation that is free of the transmitter, in the sense that the transmitter requires the same power to send changes in the field out regardless of whether anything absorbs the signal, e.g. a radio station does not need to increase its power when more receivers use the signal. This far part of the electromagnetic fieldis electromagnetic radiation. The far fields propagate (radiate) without allowing the transmitter to affect them. This causes them to be independent in the sense that their existence and their energy, after they have left the transmitter, is completely independent of both transmitter and receiver. Due toconservation of energy, the amount of power passing through any closed surface drawn around the source is the same. Thepower density of EM radiation from anisotropic source decreases with the inverse square of the distance from the source; this is called theinverse-square law. Field intensity due to dipole parts of the near field varies according to an inverse-cube law,[31] and thus fades with distance.
In theLiénard–Wiechert potential formulation of the electric and magnetic fields due to motion of a single particle (according to Maxwell's equations), the terms associated with acceleration of the particle are those that are responsible for the part of the field that is regarded as electromagnetic radiation. By contrast, the term associated with the changing static electric field of the particle and the magnetic term that results from the particle's uniform velocity are both associated with the near field, and do not comprise electromagnetic radiation.[32]
An anomaly arose in the late 19th century involving a contradiction between the wave theory of light and measurements of the electromagnetic spectra that were being emitted by thermal radiators known asblack bodies. Physicists struggled with this problem unsuccessfully for many years, and it later became known as theultraviolet catastrophe. In 1900,Max Planck developed a new theory ofblack-body radiation that explained the observed spectrum. Planck's theory was based on the idea that black bodies emit light (and other electromagnetic radiation) only as discrete bundles or packets of energy. These packets were calledquanta. In 1905,Albert Einstein proposed that light quanta be regarded as real particles. Later the particle of light was given the namephoton, to correspond with other particles being described around this time, such as theelectron andproton. A photon has an energy,E, proportional to its frequency,f, by
whereh is thePlanck constant, is the wavelength andc is thespeed of light. This is sometimes known as thePlanck–Einstein equation.[33] In quantum theory (seefirst quantization) the energy of the photons is thus directly proportional to the frequency of the EMR wave.[34] Likewise, the momentump of a photon is also proportional to its frequency and inversely proportional to its wavelength:
The source of Einstein's proposal that light was composed of particles (or could act as particles in some circumstances) was an experimental anomaly not explained by the wave theory: thephotoelectric effect, in which light striking a metal surface ejected electrons from the surface, causing anelectric current to flow across an appliedvoltage. Experimental measurements demonstrated that the energy of individual ejected electrons was proportional to thefrequency, rather than theintensity, of the light. Furthermore, below a certain minimum frequency, which depended on the particular metal, no current would flow regardless of the intensity. These observations appeared to contradict the wave theory, and for years physicists tried to find an explanation. In 1905, Einstein explained this phenomenon by resurrecting the particle theory of light. Because of the preponderance of evidence in favor of the wave theory, however, Einstein's ideas were met initially with great skepticism among established physicists. Eventually Einstein's explanation was accepted as new particle-like behavior of light was observed, such as theCompton effect.[35][36]
As a photon is absorbed by anatom, itexcites the atom, elevating an electron to a higherenergy level (one that is on average farther from the nucleus). When an electron in an excited molecule or atom descends to a lower energy level, it emits a photon of light at a frequency corresponding to the energy difference. Since the energy levels of electrons in atoms are discrete, each element and each molecule emits and absorbs its own characteristic frequencies. Immediate photon emission is calledfluorescence, a type ofphotoluminescence. An example is visible light emitted from fluorescent paints, in response to ultraviolet (blacklight). Many other fluorescent emissions are known in spectral bands other than visible light. Delayed emission is calledphosphorescence.[37][38]
Quantum mechanics also governsemission, which is seen when an emitting gas glows due to excitation of the atoms from any mechanism, including heat. As electrons descend to lower energy levels, a spectrum is emitted that represents the jumps between the energy levels of the electrons, but lines are seen because again emission happens only at particular energies after excitation.[39] An example is the emission spectrum ofnebulae.[40] Rapidly moving electrons are most sharply accelerated when they encounter a region of force, so they are responsible for producing much of the highest frequency electromagnetic radiation observed in nature. These phenomena can be used to detect the composition of gases lit from behind (absorption spectra) and for glowing gases (emission spectra).Spectroscopy (for example) determines whatchemical elements comprise a particular star. Shifts in the frequency of the spectral lines for an element, called aredshift, can be used to determine the star'scosmological distance.[41]: 181
The modern theory that explains the nature of light includes the notion of wave–particle duality. The theory is based on the concept that every quantum entity can show wave-like or particle-like behaviors, depending on observation. The observation led to the collapse of the entity'swave function. If it is based on theCopenhagen interpretation, the observation does really collapse the wave function; for themany-worlds interpretation, all possible outcomes of the collapse happened inparallel universes; for thepilot wave theory, the particle behaviour is simply determined by waves. The duality nature of a real photon has been observed in thedouble-slit experiment.
Together, wave and particle effects fully explain the emission and absorption spectra of EM radiation. The matter-composition of the medium through which the light travels determines the nature of the absorption and emission spectrum. These bands correspond to the allowed energy levels in the atoms. Dark bands in theabsorption spectrum are due to the atoms in an intervening medium between source and observer. The atoms absorb certain frequencies of the light between emitter and detector/eye, then emit them in all directions. A dark band appears to the detector, due to the radiation scattered out of thelight beam. For instance, dark bands in the light emitted by a distantstar are due to the atoms in the star's atmosphere.
In empty space (vacuum), electromagnetic radiation travels at thespeed of light,, 299,792,458 meters per second (approximately 186,000 miles per second). In a medium other than vacuum it travels at a lower velocity, given by a dimensionless parameter between 0 and 1 characteristic of the medium called thevelocity factor or its reciprocal, therefractive index:
.
The reason for this is that in matter the electric and magnetic fields of the wave are slowed because they polarize the charged particles in the medium they pass through.[42]: 401 The oscillating electric field causes nearby positive and negative charges in atoms to move slightly apart and together, inducing an oscillatingpolarization, creating an electric polarization field. The oscillating magnetic field moves nearbymagnetic dipoles, inducing an oscillatingmagnetization, creating an induced oscillating magnetic field. These induced fields,superposed on the original wave fields, slow the wave (Ewald–Oseen extinction theorem). The amount of slowing depends on the electromagnetic properties of the medium, theelectric permittivity andmagnetic permeability. In theSI system of units, empty space has avacuum permittivity of 8.854×10−12 F/m (farads per meter) and avacuum permeability of 1.257×10−6 H/m (henries per meter). These universal constants determine the speed of light in a vacuum:
In a medium that is isotropic and linear, which means the electric polarization is proportional to the electric field and the magnetization is proportional to the magnetic field. The speed of the waves, the, and the refractive index are determined by only two parameters: theelectric permittivity of the medium in farads per meter, and themagnetic permeability of the medium in henrys per meter[42]: 401
If the permittivity and permeability of the medium is constant for different frequency EM waves, this is called anon-dispersive medium.[42]: 417–418 In this case all EM wave frequencies would travel at the same velocity, and the waveshape stays constant as it travels. However in real matter and typically vary with frequency, this is called adispersive medium. In dispersive media different spectral bands have different propagation characteristics, and an arbitrary wave changes shape as it travels through the medium.
Electromagnetic radiation of wavelengths other than those of visible light were discovered in the early 19th century. The discovery ofinfrared radiation is ascribed to astronomerWilliam Herschel, who published his results in 1800 before theRoyal Society of London.[43] Herschel used a glassprism torefract light from theSun and detected invisible rays that caused heating beyond the red part of the spectrum, through an increase in the temperature recorded with athermometer. These "calorific rays" were later termed infrared.[44]
In 1801 German physicistJohann Wilhelm Ritter discoveredultraviolet in an experiment similar to Herschel's, using sunlight and a glass prism. Ritter noted that invisible rays near the violet edge of a solar spectrum dispersed by a triangular prism darkenedsilver chloride preparations more quickly than did the nearby violet light. Ritter's experiments were an early precursor to what would become photography. Ritter noted that the ultraviolet rays (which at first were called "chemical rays") were capable of causing chemical reactions.[45][46]
In 1862–64James Clerk Maxwell developed equations for the electromagnetic field which suggested that waves in the field would travel with a speed that was very close to the known speed of light. Maxwell therefore suggested that visible light (as well as invisible infrared and ultraviolet rays by inference) all consisted of propagating disturbances (or radiation) in the electromagnetic field. Radio waves were first produced deliberately byHeinrich Hertz in 1887, using electrical circuits calculated to produce oscillations at a much lower frequency than that of visible light, following recipes for producing oscillating charges and currents suggested by Maxwell's equations. Hertz also developed ways to detect these waves, and produced and characterized what were later termedradio waves andmicrowaves.[47]: 286, 7
Wilhelm Röntgen discovered and namedX-rays. After experimenting with high voltages applied to an evacuated tube on 8 November 1895, he noticed a fluorescence on a nearby plate of coated glass. In one month, he discovered X-rays' main properties.[47]: 307
The last portion of the EM spectrum to be discovered was associated withradioactivity.Henri Becquerel found thaturanium salts caused fogging of an unexposed photographic plate through a covering paper in a manner similar to X-rays, andMarie Curie discovered that only certain elements gave off these rays of energy, soon discovering the intense radiation ofradium. The radiation frompitchblende was differentiated into alpha rays (alpha particles) and beta rays (beta particles) byErnest Rutherford through simple experimentation in 1899, but these proved to be charged particulate types of radiation. However, in 1900 the French scientistPaul Villard discovered a third neutrally charged and especially penetrating type of radiation from radium, and after he described it, Rutherford realized it must be yet a third type of radiation, which in 1903 Rutherford namedgamma rays.
In 1910 British physicistWilliam Henry Bragg demonstrated that gamma rays are electromagnetic radiation, not particles, and in 1914 Rutherford andEdward Andrade measured their wavelengths, finding that they were similar to X-rays but with shorter wavelengths and higher frequency, although a 'cross-over' between X and gamma rays makes it possible to have X-rays with a higher energy (and hence shorter wavelength) than gamma rays and vice versa. The origin of the ray differentiates them, gamma rays tend to be natural phenomena originating from the unstable nucleus of an atom and X-rays are electrically generated (and hence man-made) unless they are as a result ofbremsstrahlung X-radiation caused by the interaction of fast moving particles (such as beta particles) colliding with certain materials, usually of higher atomic numbers.[47]: 308, 9
Electromagnetic spectrum with visible light highlighted. The bottom graph (visible spectrum) shows wavelength in units of nanometers (nm).Legend: γ =Gamma rays
For certain classes of EM waves, the waveform is most usefully treated asrandom, and then spectral analysis must be done by slightly different mathematical techniques appropriate to random orstochastic processes. In such cases, the individual frequency components are represented in terms of theirpower content, and the phase information is not preserved. Such a representation is called thepower spectral density of the random process. Random electromagnetic radiation requiring this kind of analysis is, for example, encountered in the interior of stars, and in certain other very wideband forms of radiation such as thezero-point wave field of the electromagnetic vacuum.
The behavior of EM radiation and its interaction with matter depends on its frequency, and changes qualitatively as the frequency changes. Lower frequencies have longer wavelengths, and higher frequencies have shorter wavelengths, and are associated with photons of higher energy. There is no fundamental limit known to these wavelengths or energies, at either end of the spectrum, although photons with energies near thePlanck energy or exceeding it (far too high to have ever been observed) will require new physical theories to describe.
Electromagnetic radiation phenomena with wavelengths ranging from one meter to one millimeter are called microwaves; with frequencies between 300 MHz (0.3 GHz) and 300 GHz. When radio waves impinge upon aconductor, they couple to the conductor, travel along it, andinduce an electric current on the conductor surface by moving the electrons of the conducting material in correlated bunches of charge. At radio and microwave frequencies, EMR interacts with matter largely as a bulk collection of charges which are spread out over large numbers of affected atoms. Inelectrical conductors, such induced bulk movement of charges (electric currents) results in absorption of the EMR, or else separations of charges that cause generation of new EMR (effective reflection of the EMR). An example is absorption or emission of radio waves by antennas, or absorption of microwaves by water or other molecules with anelectric dipole moment, as for example inside amicrowave oven. These interactions produce either electric currents or heat, or both.
Like radio and microwave, infrared (IR) is reflected by metals (and also most EMR, well into the ultraviolet range). However, unlike lower-frequency radio and microwave radiation, infrared EMR commonly interacts with dipoles present in single molecules, which change as atoms vibrate at the ends of a single chemical bond. It is consequently absorbed by a wide range of substances, causing them to increase in temperature as the vibrations dissipate as heat. The same process, run in reverse, causes bulk substances to radiate in the infrared spontaneously (seethermal radiation section below).
Infrared radiation is divided into spectral subregions. While different subdivision schemes exist,[48][49] the spectrum is commonly divided as near-infrared (0.75–1.4 μm), short-wavelength infrared (1.4–3 μm), mid-wavelength infrared (3–8 μm), long-wavelength infrared (8–15 μm) andfar infrared (15–1000 μm).[50]
Some animals, such assnakes, have thermo-sensitive membranes (pit organs) that can detect temperature differences, allowing them to sense infrared radiation.[51]
Natural sources produce EM radiation across the spectrum. EM radiation with awavelength between approximately 400nm and 700 nm is directly detected by thehuman eye and perceived as visible light. Other wavelengths, especially nearby infrared (longer than 700 nm) and ultraviolet (shorter than 400 nm) are also sometimes referred to as light.
As frequency increases into the visible range, photons have enough energy to change the bond structure of some individual molecules. It is not a coincidence that this happens in the visible range, as themechanism of vision involves the change in bonding of a single molecule,retinal, which absorbs a single photon. The change in retinal causes a change in the shape of therhodopsin protein it is contained in, which starts the biochemical process that causes theretina of the human eye to sense the light.
Visible light is able to affect only a tiny percentage of all molecules. Usually not in a permanent or damaging way, rather the photon excites an electron which then emits another photon when returning to its original position. This is the source of color produced by most dyes. Retinal is an exception. When a photon is absorbed, theretinal permanently changes structure from cis to trans, and requires a protein to convert it back, i.e. reset it to be able to function as a light detector again.
Photosynthesis becomes possible in this range as well, for the same reason. A single molecule ofchlorophyll is excited by a single photon. In plant tissues that conduct photosynthesis,carotenoids act to quench electronically excited chlorophyll produced by visible light in a process callednon-photochemical quenching, to prevent reactions that would otherwise interfere with photosynthesis at high light levels.
Infrared, microwaves, and radio waves are known to damage molecules and biological tissue only by bulk heating, not excitation from single photons of the radiation.
As frequency increases into the ultraviolet, photons now carry enough energy (about threeelectron volts or more) to excite certain doubly bonded molecules into permanent chemical rearrangement. InDNA, this causes lasting damage. DNA is also indirectly damaged by reactive oxygen species produced by ultraviolet A (UVA), which has energy too low to damage DNA directly. This is why ultraviolet at all wavelengths can damage DNA, and is capable of causing cancer, and (forUVB) skin burns (sunburn) that are far worse than would be produced by simple heating (temperature increase) effects.
At the higher end of the ultraviolet range, the energy of photons becomes large enough to impart enough energy to electrons to cause them to be liberated from the atom, in a process calledphotoionisation. The energy required for this is always larger than about 10electron volt (eV) corresponding with wavelengths smaller than 124 nm (some sources suggest a more realistic cutoff of 33 eV, which is the energy required to ionize water). This high end of the ultraviolet spectrum with energies in the approximate ionization range, is sometimes called "extreme UV". Ionizing UV is strongly filtered by the Earth's atmosphere.[53]
Electromagnetic radiation composed of photons that carry minimum-ionization energy, or more (which includes the entire spectrum with shorter wavelengths), is therefore termedionizing radiation. (Many other kinds of ionizing radiation are made of non-EM particles.) Electromagnetic-type ionizing radiation extends from the extreme ultraviolet to all higher frequencies and shorter wavelengths, which means that all X-rays and gamma rays qualify. These are capable of the most severe types of molecular damage, which can happen in biology to any type of biomolecule, including mutation and cancer,[54] and often at great depths below the skin, since the higher end of the X-ray spectrum, and all of the gamma ray spectrum, penetrate matter.
Rough plot of Earth's atmospheric absorption and scattering (oropacity) of variouswavelengths of electromagnetic radiation
Most UV and X-rays are blocked by absorption first from molecularnitrogen, and then (for wavelengths in the upper UV) from the electronic excitation ofdioxygen and finallyozone at the mid-range of UV. Only 30% of the Sun's ultraviolet light reaches the ground, and almost all of this is well transmitted.
Visible light is well transmitted in air, a property known as anatmospheric window, as it is not energetic enough to excite nitrogen, oxygen, or ozone, but too energetic to excite molecular vibrational frequencies of water vapor and carbon dioxide.[55] Absorption bands in the infrared are due to modes of vibrational excitation in water vapor. However, at energies too low to excite water vapor, the atmosphere becomes transparent again, allowing free transmission of most microwave and radio waves.[56]
Finally, at radio wavelengths longer than 10 m or so (about 30 MHz), the air in the lower atmosphere remains transparent to radio, but plasma in certain layers of theionosphere begins to interact with radio waves (seeskywave). This property allows some longer wavelengths (100 m or 3 MHz) to be reflected and results inshortwave radio beyond line-of-sight. However, certainionospheric effects begin to block incoming radiowaves from space, when their frequency is less than about 10 MHz (wavelength longer than about 30 m).[57]
Thermal and electromagnetic radiation as a form of heat
The basic structure ofmatter involves charged particles bound together. When electromagnetic radiation impinges on matter, it causes the charged particles to oscillate and gain energy. The ultimate fate of this energy depends on the context. It could be immediately re-radiated and appear as scattered, reflected, or transmitted radiation. It may get dissipated into other microscopic motions within the matter, coming tothermal equilibrium and manifesting itself asthermal energy, or evenkinetic energy, in the material. With a few exceptions related to high-energy photons (such asfluorescence,harmonic generation,photochemical reactions, thephotovoltaic effect for ionizing radiations at far ultraviolet, X-ray, and gamma radiation), absorbed electromagnetic radiation simply deposits its energy by heating the material. This happens for infrared, microwave, and radio wave radiation.
Intense radio waves can thermally burn living tissue and can cook food. In addition to infraredlasers, sufficiently intense visible and ultraviolet lasers can easily set paper afire.[58] Ionizing radiation creates high-speed electrons in a material and breaks chemical bonds, but after these electrons collide many times with other atoms eventually most of the energy becomes thermal energy all in a tiny fraction of a second. This caveat also applies to UV, even though almost all of it is not ionizing, because UV can damage molecules due to electronic excitation, which is far greater per unit energy than heating effects.[58][54]
Infrared radiation in the spectral distribution of ablack body is usually considered a form of heat, since it has an equivalent temperature and is associated with an entropy change per unit of thermal energy. However, "heat" is a technical term in physics and thermodynamics and is often confused with thermal energy. Any type of electromagnetic energy can be transformed into thermal energy in interaction with matter. Thus,any electromagnetic radiation can "heat" (in the sense of increase thethermal energy temperature of) a material, when it is absorbed.[59] The inverse or time-reversed process of absorption is thermal radiation. Much of the thermal energy in matter consists of random motion of charged particles, and this energy can be radiated away from the matter. The resulting radiation may subsequently be absorbed by another piece of matter, with the deposited energy heating the material.[60]
The electromagnetic radiation in an opaque cavity at thermal equilibrium is effectively a form of thermal energy, having maximumradiation entropy.[61]
Bioelectromagnetics is the study of the interactions and effects of EM radiation on living organisms. The effects of electromagnetic radiation upon living cells, including those in humans, depends upon the radiation's power and frequency. For low-frequency radiation (radio waves to near ultraviolet) the best-understood effects are those due to radiation power alone, acting through heating when radiation is absorbed. For these thermal effects, frequency is important as it affects the intensity of the radiation and penetration into the organism (for example, microwaves penetrate better than infrared). It is widely accepted that low frequency fields that are too weak to cause significant heating could not possibly have any biological effect.[62] Some research suggests that weakernon-thermal electromagnetic fields (including weak ELF magnetic fields, although the latter does not strictly qualify as EM radiation[62][63][64]) and modulated RF and microwave fields can have biological effects, though the significance of this is unclear.[65][66]
TheWorld Health Organization has classified radio frequency electromagnetic radiation asGroup 2B—possibly carcinogenic.[67][68] This group contains possible carcinogens such as lead, DDT, and styrene. At higher frequencies (some of visible and beyond), the effects of individual photons begin to become important, as these now have enough energy individually to directly or indirectly damage biological molecules.[69] All UV frequencies have been classed as Group 1 carcinogens by the World Health Organization. Ultraviolet radiation from sun exposure is the primary cause of skin cancer.[70][71]
Thus, at UV frequencies and higher, electromagnetic radiation does more damage to biological systems than simple heating predicts. This is most obvious in the "far" (or "extreme") ultraviolet. UV, with X-ray and gamma radiation, are referred to asionizing radiation due to the ability of photons of this radiation to produceions andfree radicals in materials (including living tissue). Since such radiation can severely damage life at energy levels that produce little heating, it is considered far more dangerous (in terms of damage-produced per unit of energy, or power) than the rest of the electromagnetic spectrum.
The heat ray is an application of EMR that makes use of microwave frequencies to create an unpleasant heating effect in the upper layer of the skin. A publicly known heat ray weapon called theActive Denial System was developed by the US military as an experimental weapon to deny the enemy access to an area.[72] Adeath ray is a theoretical weapon that delivers heat ray based on electromagnetic energy at levels that are capable of injuring human tissue. An inventor of a death ray,Harry Grindell Matthews, claimed to have lost sight in his left eye while working on his death ray weapon based on a microwavemagnetron from the 1920s (a normalmicrowave oven creates a tissue damaging cooking effect inside the oven at around 2 kV/m).[73]
Electromagnetic waves are predicted by the classical laws of electricity and magnetism, known asMaxwell's equations. There are nontrivial solutions of the homogeneous Maxwell's equations (without charges or currents), describingwaves of changing electric and magnetic fields. Beginning with Maxwell's equations infree space:
Besides the trivial solution, useful solutions can be derived with the followingvector identity, valid for all vectors in some vector field:Taking the curl of the second Maxwell's equation (2) yields:
5
Evaluating the left hand side of (5) with the above identity and simplifying using (1), yields:
6
Evaluating the right hand side of (5) by exchanging the sequence of derivatives and inserting the fourthMaxwell's equation (4), yields:
7
Combining (6) and (7) again, gives a vector-valueddifferential equation for the electric field, solving the homogeneous Maxwell's equations:
Taking the curl of the fourth Maxwell's equation (4) results in a similar differential equation for a magnetic field solving the homogeneous Maxwell's equations:
Both differential equations have the form of the generalwave equation for waves propagating with speed where is a function of time and location, which gives the amplitude of the wave at some time at a certain location:This is also written as:where denotes the so-calledd'Alembert operator, which in Cartesian coordinates is given as:
Comparing the terms for the speed of propagation, yields in the case of the electric and magnetic fields:
This is thespeed of light in vacuum. Thus Maxwell's equations connect thevacuum permittivity, thevacuum permeability, and the speed of light,c0, via the above equation. This relationship had been discovered byWilhelm Eduard Weber andRudolf Kohlrausch prior to the development of Maxwell's electrodynamics, however Maxwell was the first to produce a field theory consistent with waves traveling at the speed of light.
These are only two equations versus the original four, so more information pertains to these waves hidden within Maxwell's equations. A generic vector wave for the electric field has the formHere, is a constant vector, is any second differentiable function, is a unit vector in the direction of propagation, and is a position vector. is a generic solution to the wave equation. In other words,for a generic wave traveling in the direction.
From the first of Maxwell's equations, we getThus,which implies that the electric field is orthogonal to the direction the wave propagates. The second of Maxwell's equations yields the magnetic field, namely,Thus,The remaining equations will be satisfied by this choice of.
The electric and magnetic field waves in the far-field travel at the speed of light. They have a special restricted orientation and proportional magnitudes,, which can be seen immediately from thePoynting vector. The electric field, magnetic field, and direction of wave propagation are all orthogonal, and the wave propagates in the same direction as. AlsoE andB far-fields in free space, which as wave solutions depend primarily on these two Maxwell's equations to remain in phase with each other. This is guaranteed since the generic wave solution is first order in both space and time, and thecurl operator on one side of these equations results in first-order spatial derivatives of the wave solution, while the time-derivative on the other side of the equations, which gives the other field, is first-order in time, resulting in the samephase shift for both fields in each mathematical operation.
From the viewpoint of an electromagnetic wave traveling forward, the electric field might be oscillating up and down, while the magnetic field oscillates right and left. This picture can be rotated with the electric field oscillating right and left and the magnetic field oscillating down and up. This is a different solution that is traveling in the same direction. This arbitrariness in the orientation with respect to propagation direction is known aspolarization. On a quantum level, it is described asphoton polarization. The direction of the polarization is defined as the direction of the electric field.
More general forms of the second-order wave equations given above are available, allowing for both non-vacuum propagation media and sources. Many competing derivations exist, all with varying levels of approximation and intended applications. One very general example is a form of the electric field equation,[74] which was factorized into a pair of explicitly directional wave equations, and then efficiently reduced into a single uni-directional wave equation by means of a simple slow-evolution approximation.
^*Purcell and Morin, Harvard University. (2013).Electricity and Magnetism, 820p (3rd ed.). Cambridge University Press, New York.ISBN978-1-107-01402-2. p 430: "These waves... require no medium to support their propagation. Traveling electromagnetic waves carry energy, and... thePoynting vector describes the energy flow...;" p 440: ... the electromagnetic wave must have the following properties: 1) The field pattern travels with speed c (speed of light); 2) At every point within the wave... the electric field strength E equals "c" times the magnetic field strength B; 3) The electric field and the magnetic field are perpendicular to one another and to the direction of travel, or propagation."
^*Browne, Michael (2013).Physics for Engineering and Science, p427 (2nd ed.). McGraw Hill/Schaum, New York.ISBN978-0-07-161399-6.; p319: "For historical reasons, different portions of the EM spectrum are given different names, although they are all the same kind of thing. Visible light constitutes a narrow range of the spectrum, from wavelengths of about 400-800 nm.... ;p 320 "An electromagnetic wave carries forward momentum... If the radiation is absorbed by a surface, the momentum drops to zero and a force is exerted on the surface... Thus the radiation pressure of an electromagnetic wave is (formula)."
^Purcell, p442: "Any number of electromagnetic waves can propagate through the same region without affecting one another. The fieldE at a space time point is the vector sum of the electric fields of the individual waves, and the same goes forB".
^Browne, p 376: "Radiation is emitted or absorbed only when the electron jumps from one orbit to the other, and the frequency of radiation depends only upon on the energies of the electron in the initial and final orbits.
^abBaeyens, Ans; Abrantes, Ana Margarida; Ahire, Vidhula; Ainsbury, Elizabeth A.; Baatout, Sarah; Baselet, Bjorn; Botelho, Maria Filomena; Boterberg, Tom; Chevalier, Francois (2023), Baatout, Sarah (ed.),"Basic Concepts of Radiation Biology",Radiobiology Textbook, Cham: Springer International Publishing, pp. 25–81,doi:10.1007/978-3-031-18810-7_2,ISBN978-3-031-18810-7, retrieved24 March 2025
^Tao, Jiasheng (2023). "Radiation Source and Optical Atmospheric Transmission".Space Optical Remote Sensing. Advances in Optics and Optoelectronics. Springer, Singapore. pp. 111–188.doi:10.1007/978-981-99-3318-1_4.ISBN978-981-99-3318-1.
^Chaplin, Martin (15 May 2013)."Infared Spectroscopy"(PDF): water.lsbu.ac.uk. Archived fromthe original(PDF) on 24 March 2022. Retrieved19 April 2022.{{cite journal}}:Cite journal requires|journal= (help)
^Cleary, S. F.; Liu, L. M.; Merchant, R. E. (1990). "In vitro lymphocyte proliferation induced by radio-frequency electromagnetic radiation under isothermal conditions".Bioelectromagnetics.11 (1):47–56.doi:10.1002/bem.2250110107.PMID2346507.
Tipler, Paul (2004).Physics for Scientists and Engineers: Electricity, Magnetism, Light, and Elementary Modern Physics (5th ed.). W. H. Freeman.ISBN978-0-7167-0810-0.
Reitz, John; Milford, Frederick; Christy, Robert (1992).Foundations of Electromagnetic Theory (4th ed.). Addison Wesley.ISBN978-0-201-52624-0.
Allen Taflove and Susan C. Hagness (2005).Computational Electrodynamics: The Finite-Difference Time-Domain Method, 3rd ed. Artech House Publishers.ISBN978-1-58053-832-9.
