In physics, electromagnetic radiation (EM radiation or EMR) refers to
the waves (or their quanta, photons) of the electromagnetic field,
propagating (radiating) through space-time, carrying electromagnetic
radiant energy. It includes radio waves, microwaves, infrared,
(visible) light, ultraviolet, X-rays, and gamma rays.
Classically, electromagnetic radiation consists of electromagnetic
waves, which are synchronized oscillations of electric and magnetic
fields that propagate at the speed of light through a vacuum. The
oscillations of the two fields are perpendicular to each other and
perpendicular to the direction of energy and wave propagation, forming
a transverse wave. The wavefront of electromagnetic waves emitted from
a point source (such as a light bulb) is a sphere. The position of an
electromagnetic wave within the electromagnetic spectrum could be
characterized by either its frequency of oscillation or its
wavelength. The electromagnetic spectrum includes, in order of
increasing frequency and decreasing wavelength: radio waves,
microwaves, infrared radiation, visible light, ultraviolet radiation,
and gamma rays.
EMR are emitted from a system of electrically charged particles, and
these waves can subsequently interact with other charged particles. EM
waves carry energy, momentum and angular momentum away from their
source particle and can impart those quantities to matter with which
they interact. Quanta of EM waves are called photons, whose rest mass
is associated with those EM waves
that are free to propagate themselves ("radiate") without the
continuing influence of the moving charges that produced them, because
they have achieved sufficient distance from those charges. Thus, EMR
is sometimes referred to as the far field. In this language, the near
field refers to EM fields near the charges and current that directly
produced them specifically, electromagnetic induction and
electrostatic induction phenomena.
In the domain of classical electrodynamics, EMR satisfies the
following self-evident physical laws. EMR carries energy. Therefore,
due to the energy conservation law, every EMR must have a source of
electric charges that supplied its energy. EMR propagates in the
vacuum without losing energy. For this reason, its energy current must
abide by the inverse square law.
current of an electromagnetic
wave is described by the Poynting vector, which is proportional to the
vector product of the wave’s electric and magnetic fields. Hence,
the inverse square law proves that the strength of the EMR electric
field and that of its magnetic field must decrease like 1/R, where R
denotes the distance from the source. The energy of electromagnetic
radiation and the laws of
electromagnetic fields propagate at a finite speed, and in the vacuum,
it is the speed of light. Therefore, radiation fields measured at a
given space-time point were produced by electric charges of the source
at an earlier time (called the retarded time). Mathematically,
electromagnetic fields are obtained as spatial and temporal
derivatives of 4-potentials called the Lienard-Wiechert 4-potentials.
These 4-potentials depend on the retarded position and the retarded
velocity of the charges at the source. Referring to the distance from
the source, these 4-potentials decrease like 1/R. It follows that a
derivative of the 4-potentials with respect to a spatial coordinate,
decreases like 1/R2. This result proves that radiation fields are
obtained only from a time-derivative of the 4-potentials. Since the
4-potentials depend on the velocity of the electric charges, one finds
that due to the time-derivative, an acceleration of the charged
particles at the source is a necessary condition for radiation.
Furthermore, electromagnetic fields satisfy the wave equation.
Therefore, the actual radiation emitted from a source is determined by
the interference of the fields that are associated with the
accelerating charges at the source. (A common misconception states
that charge-acceleration is a sufficient condition for radiation. This
is not true. For example, take the electric current that flows along a
circular conductor which is connected to a battery. The ring itself is
motionless. However, charges of the electric current accelerate
towards the ring’s center. Moreover, the system is time-independent,
and for this reason it transfers no electromagnetic energy to the
environment. Hence, this system contains accelerating charged
particles, but it emits no radiation. This is an example where a
destructive interference cancels the entire radiation.)
In the quantum theory of electromagnetism, EMR consists of photons,
the elementary particles responsible for all electromagnetic
effects provide additional sources of EMR,
such as the transition of electrons to lower energy levels in an atom
and black-body radiation. The energy of an individual photon is
quantized and is greater for photons of higher frequency. This
relationship is given by Planck's equation E = hν, where E is the
energy per photon, ν is the frequency of the photon, and h is
Planck's constant. A single gamma ray photon, for example, might carry
~100,000 times the energy of a single photon of visible light.
The effects of EMR upon chemical compounds and biological organisms
depend both upon the radiation's power and its frequency. EMR of
visible or lower frequencies (i.e., visible light, infrared,
microwaves, and radio waves) is called non-ionizing radiation, because
its photons do not individually have enough energy to ionize atoms or
molecules. The effects of these radiations on chemical systems and
living tissue are caused primarily by heating effects from the
combined energy transfer of many photons. In contrast, high frequency
and gamma rays are called ionizing radiation,
since individual photons of such high frequency have enough energy to
ionize molecules or break chemical bonds. These radiations have the
ability to cause chemical reactions and damage living cells beyond
that resulting from simple heating, and can be a health hazard.
1.1.1 Maxwell’s equations
1.1.2 Near and far fields
1.3 Wave model
1.4 Particle model and quantum theory
1.5 Wave–particle duality
1.6 Wave and particle effects of electromagnetic radiation
1.7 Propagation speed
Special theory of relativity
2 History of discovery
3 Electromagnetic spectrum
3.1 Interactions as a function of frequency
3.1.1 Radio and microwave
3.1.3 Visible light
X-rays and gamma rays
4 Atmosphere and magnetosphere
5 Types and sources, classed by spectral band
5.1 Radio waves
5.4 Visible light
5.7 Gamma rays
Thermal radiation and electromagnetic radiation as a form of heat
6 Biological effects
7 Derivation from electromagnetic theory
8 See also
10 Further reading
11 External links
Shows the relative wavelengths of the electromagnetic waves of three
different colours of light (blue, green, and red) with a distance
scale in micrometers along the x-axis.
Maxwell's equations and Near and far field
Maxwell derived a wave form of the electric and magnetic equations,
thus uncovering the wave-like nature of electric and magnetic fields
and their symmetry. Because the speed of EM waves predicted by the
wave equation coincided with the measured speed of light, Maxwell
concluded that light itself is an EM wave. Maxwell’s
equations were confirmed by
Heinrich Hertz through experiments with
According to Maxwell's equations, a spatially varying electric field
is always associated with a magnetic field that changes over time.
Likewise, a spatially varying magnetic field is associated with
specific changes over time in the electric field. In an
electromagnetic wave, the changes in the electric field are always
accompanied by a wave in the magnetic field in one direction, and vice
versa. This relationship between the two occurs without either type
field causing the other; rather, they occur together in the same way
that time and space changes occur together and are interlinked in
special relativity. In fact, magnetic fields may be viewed as
relativistic distortions of electric fields, so the close relationship
between space and time changes here is more than an analogy. Together,
these fields form a propagating electromagnetic wave, which moves out
into space and need never again affect the source. The distant EM
field formed in this way by the acceleration of a charge carries
energy with it that "radiates" away through space, hence the term.
Near and far fields
Near and far field
Near and far field and Liénard–Wiechert potential
In electromagnetic radiation (such as microwaves from an antenna,
shown here) the term applies only to the parts of the electromagnetic
field that radiate into infinite space and decrease in intensity by an
inverse-square law of power, so that the total radiation energy that
crosses through an imaginary spherical surface is the same, no matter
how far away from the antenna the spherical surface is drawn.
Electromagnetic radiation thus includes the far field part of the
electromagnetic field around a transmitter. A part of the "near-field"
close to the transmitter, forms part of the changing electromagnetic
field, but does not count as electromagnetic radiation.
Maxwell's equations established that some charges and currents
("sources") produce a local type of electromagnetic field near them
that does not have the behaviour of EMR. Currents directly produce a
magnetic field, but it is of a magnetic 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 an electric dipole type electrical field, but this
also declines with distance. These fields make up the near-field near
the EMR source. Neither of these behaviours are responsible for EM
radiation. Instead, they cause electromagnetic field behaviour that
only efficiently transfers power to a receiver very close to the
source, such as the magnetic induction inside a transformer, or the
feedback behaviour that happens close to the coil of a metal detector.
Typically, near-fields have a powerful effect on their own sources,
causing an increased “load” (decreased electrical reactance) in
the source or transmitter, whenever energy is withdrawn from the EM
field by a receiver. Otherwise, these fields do not “propagate”
freely out into space, carrying their energy away without
distance-limit, but rather oscillate, returning their energy to the
transmitter if it is not received by a receiver.
By contrast, the EM far-field is composed of radiation that is free of
the transmitter in the sense that (unlike the case in an electrical
transformer) the transmitter requires the same power to send these
changes in the fields out, whether the signal is immediately picked up
or not. This distant part of the electromagnetic field is
"electromagnetic radiation" (also called the far-field). 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 to conservation of energy, the amount of power passing
through any spherical surface drawn around the source is the same.
Because such a surface has an area proportional to the square of its
distance from the source, the power density of EM radiation always
decreases with the inverse square of distance from the source; this is
called the inverse-square law. This is in contrast to dipole parts of
the EM field close to the source (the near-field), which varies in
power according to an inverse cube power law, and thus does not
transport a conserved amount of energy over distances, but instead
fades with distance, with its energy (as noted) rapidly returning to
the transmitter or absorbed by a nearby receiver (such as a
transformer secondary coil).
The far-field (EMR) depends on a different mechanism for its
production than the near-field, and upon different terms in
Maxwell’s equations. Whereas the magnetic part of the near-field is
due to currents in the source, the magnetic field in EMR is due only
to the local change in the electric field. In a similar way, while the
electric field in the near-field is due directly to the charges and
charge-separation in the source, the electric field in EMR is due to a
change in the local magnetic field. Both processes for producing
electric and magnetic EMR fields have a different dependence on
distance than do near-field dipole electric and magnetic fields. That
is why the EMR type of EM field becomes dominant in power “far”
from sources. The term “far from sources” refers to how far from
the source (moving at the speed of light) any portion of the
outward-moving EM field is located, by the time that source currents
are changed by the varying source potential, and the source has
therefore begun to generate an outwardly moving EM field of a
different phase.
A more compact view of EMR is that the far-field that composes EMR is
generally that part of the EM field that has traveled sufficient
distance from the source, that it has become completely disconnected
from any feedback to the charges and currents that were originally
responsible for it. Now independent of the source charges, the EM
field, as it moves farther away, is dependent only upon the
accelerations of the charges that produced it. It no longer has a
strong connection to the direct fields of the charges, or to the
velocity of the charges (currents).
Liénard–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 electromagnetic near-field, and do not
comprise EM radiation.
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.
Note that the electric and magnetic fields in such a wave are in-phase
with each other, reaching minima and maxima together
An alternate view of the wave shown above.
Electrodynamics is the physics of electromagnetic radiation, and
electromagnetism is the physical phenomenon associated with the theory
of electrodynamics. Electric and magnetic fields obey the properties
of superposition. 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 are
vector fields, all magnetic and electric field vectors add together
according to vector addition. For example, in optics two or more
coherent lightwaves may interact and by constructive or destructive
interference yield a resultant irradiance deviating from the sum of
the component irradiances of the individual lightwaves.[citation
Since light is an oscillation it is not affected by traveling through
static electric or magnetic fields in a linear medium such as a
vacuum. However, in nonlinear media, such as some crystals,
interactions can occur between light and static electric and magnetic
fields — these interactions include the
Faraday effect and the Kerr
In refraction, a wave crossing from one medium to another of different
density alters its speed and direction upon entering the new medium.
The ratio of the refractive indices of the media determines the degree
of refraction, and is summarized by Snell's law.
Light of composite
wavelengths (natural sunlight) disperses into a visible spectrum
passing through a prism, because of the wavelength-dependent
refractive index of the prism material (dispersion); that is, each
component wave within the composite light is bent a different
EM radiation exhibits both wave properties and particle properties at
the same time (see wave-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 too 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 of matter. Demonstrating that the light itself
is quantized, not merely its interaction with matter, is a more subtle
Some experiments display both the wave and particle natures of
electromagnetic waves, such as the self-interference of a single
photon. When a single photon is sent through an interferometer, it
passes through both paths, interfering with itself, as waves do, yet
is detected by a photomultiplier or other sensitive detector only
A quantum theory of the interaction between electromagnetic radiation
and matter such as electrons is described by the theory of quantum
Electromagnetic waves can be polarized, reflected, refracted,
diffracted or interfere with each other.
Representation of the electric field vector of a wave of circularly
polarized electromagnetic radiation.
Electromagnetic radiation is a transverse wave, meaning that its
oscillations are perpendicular to the direction of energy transfer and
travel. The electric and magnetic parts of the field stand in a
fixed ratio of strengths in order to satisfy the two Maxwell equations
that specify how one is produced from the other. These E and B fields
are also in phase, with both reaching maxima and minima at the same
points in space (see illustrations). A common misconception is that
the E and B fields in electromagnetic radiation are out of phase
because a change in one produces the other, and this would produce a
phase difference between them as sinusoidal functions (as indeed
happens in electromagnetic induction, and in the near-field close to
antennas). However, in the far-field EM radiation which is described
by the two source-free Maxwell curl operator equations, a more correct
description is that a time-change in one type of field is proportional
to a space-change in the other. These derivatives require that the E
and B fields in EMR are in-phase (see math section below).[citation
An important aspect of light's nature is its frequency. The frequency
of a wave is its rate of oscillation and is measured in hertz, the SI
unit of frequency, where one hertz is equal to one oscillation per
Light usually has multiple frequencies that sum to form the
resultant wave. Different frequencies undergo different angles of
refraction, a phenomenon known as dispersion.
A wave consists of successive troughs and crests, and the distance
between two adjacent crests or troughs is called the wavelength. Waves
of the electromagnetic spectrum vary in size, from very long radio
waves the size of buildings to very short gamma rays smaller than atom
Frequency is inversely proportional to wavelength, according
to the equation:
displaystyle displaystyle v=flambda
where v is the speed of the wave (c in a vacuum, or less in other
media), f is the frequency and λ 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's
electromagnetic 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 of Fourier analysis
into its frequency spectrum, or individual sinusoidal components, each
of which contains a single frequency, amplitude and phase. Such a
component wave is said to be monochromatic. A monochromatic
electromagnetic wave can be characterized by its frequency or
wavelength, its peak amplitude, its phase relative to some reference
phase, its direction of propagation and its polarization.
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. An example of interference caused by EMR is
electromagnetic interference (EMI) or as it is more commonly known as,
radio-frequency interference (RFI). Additionally,
multiple polarization signals can be combined (i.e. interfered) to
form new states of polarization, which is known as parallel
polarization state generation.
The energy in electromagnetic waves is sometimes called radiant
Particle model and quantum theory
Quantization (physics) and
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 as black bodies. Physicists struggled with this problem, which
later became known as the ultraviolet catastrophe, unsuccessfully for
many years. In 1900,
Max Planck developed a new theory of black-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 called quanta. Later, Albert Einstein
proposed that light quanta be regarded as real particles. Later the
particle of light was given the name photon, to correspond with other
particles being described around this time, such as the electron and
proton. A photon has an energy, E, proportional to its frequency, f,
displaystyle E=hf= frac hc lambda ,!
where h is Planck's constant,
is the wavelength and c is the speed of light. This is sometimes
known as the Planck–Einstein equation. In quantum theory (see
first quantization) the energy of the photons is thus directly
proportional to the frequency of the EMR wave.
Likewise, the momentum p of a photon is also proportional to its
frequency and inversely proportional to its wavelength:
displaystyle p= E over c = hf over c = h over lambda .
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: the photoelectric effect, in
which light striking a metal surface ejected electrons from the
surface, causing an electric current to flow across an applied
voltage. Experimental measurements demonstrated that the energy of
individual ejected electrons was proportional to the frequency, rather
than the intensity, 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 in vain to
find an explanation. In 1905, Einstein explained this puzzle by
resurrecting the particle theory of light to explain the observed
effect. 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 the Compton effect.
As a photon is absorbed by an atom, it excites the atom, elevating an
electron to a higher energy 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 called fluorescence, a type of photoluminescence. 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
called phosphorescence.
Main article: Wave-particle duality
The modern theory that explains the nature of light includes the
notion of wave–particle duality. More generally, the theory states
that everything has both a particle nature and a wave nature, and
various experiments can be done to bring out one or the other. The
particle nature is more easily discerned using an object with a large
mass. A bold proposition by
Louis de Broglie
Louis de Broglie in 1924 led the
scientific community to realize that electrons also exhibited
Wave and particle effects of electromagnetic radiation
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 the absorption
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 the beam. For instance, dark bands in the light emitted by a
distant star are due to the atoms in the star's atmosphere. A similar
phenomenon occurs for emission, 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. An example is the emission
spectrum of nebulae. 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 aid various chemical determinations for the
composition of gases lit from behind (absorption spectra) and for
glowing gases (emission spectra). Spectroscopy (for example)
determines what chemical elements comprise a particular star.
Spectroscopy is also used in the determination of the distance of a
star, using the red shift.
Main article: Speed of light
When any wire (or other conducting object such as an antenna) conducts
alternating current, electromagnetic radiation is propagated at the
same frequency as the current. In many such situations it is possible
to identify an electrical dipole moment that arises from separation of
charges due to the exciting electrical potential, and this dipole
moment oscillates in time, as the charges move back and forth. This
oscillation at a given frequency gives rise to changing electric and
magnetic fields, which then set the electromagnetic radiation in
At the quantum level, electromagnetic radiation is produced when the
wavepacket of a charged particle oscillates or otherwise accelerates.
Charged particles in a stationary state do not move, but a
superposition of such states may result in a transition state that has
an electric dipole moment that oscillates in time. This oscillating
dipole moment is responsible for the phenomenon of radiative
transition between quantum states of a charged particle. Such states
occur (for example) in atoms when photons are radiated as the atom
shifts from one stationary state to another.
As a wave, light is characterized by a velocity (the speed of light),
wavelength, and frequency. As particles, light is a stream of photons.
Each has an energy related to the frequency of the wave given by
Planck's relation E = hf, where E is the energy of the photon, h =
6.626 × 10−34 J·s is Planck's constant, and f is the frequency of
the wave.
One rule is obeyed regardless of circumstances: EM radiation in a
vacuum travels at the speed of light, relative to the observer,
regardless of the observer's velocity. (This observation led to
Einstein's development of the theory of special relativity.)[citation
In a medium (other than vacuum), velocity factor or refractive index
are considered, depending on frequency and application. Both of these
are ratios of the speed in a medium to speed in a vacuum.[citation
Special theory of relativity
Special theory of relativity
By the late nineteenth century, various experimental anomalies could
not be explained by the simple wave theory. One of these anomalies
involved a controversy over the speed of light. The speed of light and
other EMR predicted by
Maxwell's equations did not appear unless the
equations were modified in a way first suggested by FitzGerald and
Lorentz (see history of special relativity), or else otherwise that
speed would depend on the speed of observer relative to the "medium"
(called luminiferous aether) which supposedly "carried" the
electromagnetic wave (in a manner analogous to the way air carries
sound waves). Experiments failed to find any observer effect. In 1905,
Einstein proposed that space and time appeared to be
velocity-changeable entities for light propagation and all other
processes and laws. These changes accounted for the constancy of the
speed of light and all electromagnetic radiation, from the viewpoints
of all observers—even those in relative motion.
History of discovery
History of electromagnetic theory
History of electromagnetic theory and Timeline of
Electromagnetic radiation of wavelengths other than those of visible
light were discovered in the early 19th century. The discovery of
infrared radiation is ascribed to astronomer William Herschel, who
published his results in 1800 before the Royal Society of London.
Herschel used a glass prism to refract light from the
Sun and detected
invisible rays that caused heating beyond the red part of the
spectrum, through an increase in the temperature recorded with a
thermometer. These "calorific rays" were later termed
In 1801, German physicist
Johann Wilhelm Ritter
Johann Wilhelm Ritter discovered ultraviolet
in an experiment similar to Hershel'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 darkened silver
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
James Clerk Maxwell
James 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 by
Heinrich 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
Hertz also developed ways to detect these waves, and
produced and characterized what were later termed radio waves and
Wilhelm Röntgen discovered and named X-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.:307
The last portion of the EM spectrum to be discovered was associated
Henri Becquerel found that uranium salts caused
fogging of an unexposed photographic plate through a covering paper in
a manner similar to X-rays, and
Marie Curie discovered that only
certain elements gave off these rays of energy, soon discovering the
intense radiation of radium. The radiation from pitchblende was
differentiated into alpha rays (alpha particles) and beta rays (beta
Ernest Rutherford through simple experimentation in
1899, but these proved to be charged particulate types of radiation.
However, in 1900 the French scientist
Paul 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 named gamma rays.
In 1910 British physicist
William Henry Bragg
William Henry Bragg demonstrated that gamma
rays are electromagnetic radiation, not particles, and in 1914
Edward 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 a natural phenomena
originating from the unstable nucleus of an atom and
electrically generated (and hence man-made) unless they are as a
result of bremsstrahlung X-radiation caused by the interaction of fast
moving particles (such as beta particles) colliding with certain
materials, usually of higher atomic numbers.:308,9
Electromagnetic spectrum with visible light highlighted
Main article: Electromagnetic spectrum
γ = Gamma rays
HX = Hard X-rays
SX = Soft X-Rays
EUV = Extreme-ultraviolet
NUV = Near-ultraviolet
Visible light (colored bands)
NIR = Near-infrared
MIR = Mid-infrared
FIR = Far-infrared
Extremely high frequency
Extremely high frequency (microwaves)
Super-high frequency (microwaves)
Ultrahigh frequency (radio waves)
Very high frequency
Very high frequency (radio)
High frequency (radio)
Medium frequency (radio)
Low frequency (radio)
Very low frequency
Very low frequency (radio)
VF = Voice frequency
Ultra-low frequency (radio)
Super-low frequency (radio)
ELF = Extremely low frequency(radio)
EM radiation (the designation 'radiation' excludes static electric and
magnetic and near fields) is classified by wavelength into radio,
microwave, infrared, visible, ultraviolet,
X-rays and gamma rays.
Arbitrary electromagnetic waves can be expressed by Fourier analysis
in terms of sinusoidal monochromatic waves, which in turn can each be
classified into these regions of the EMR spectrum.
For certain classes of EM waves, the waveform is most usefully treated
as random, and then spectral analysis must be done by slightly
different mathematical techniques appropriate to random or stochastic
processes. In such cases, the individual frequency components are
represented in terms of their power content, and the phase information
is not preserved. Such a representation is called the power 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 the Zero point wave field of the electromagnetic
The behavior of EM radiation depends on its frequency. 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 the
Planck energy or exceeding it (far too high to have ever been
observed) will require new physical theories to describe.
Soundwaves are not electromagnetic radiation. At the lower end of the
electromagnetic spectrum, about 20 Hz to about 20 kHz, are
frequencies that might be considered in the audio range. However,
electromagnetic waves cannot be directly perceived by human ears.
Sound waves are instead the oscillating compression of molecules. To
be heard, electromagnetic radiation must be converted to pressure
waves of the fluid in which the ear is located (whether the fluid is
air, water or something else).
Interactions as a function of frequency
When EM radiation interacts with matter, its behavior changes
qualitatively as its frequency changes.
Radio and microwave
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. In electrical 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 an electric dipole moment, as for
example inside a microwave oven. These interactions produce either
electric currents or heat, or both.
Like radio and microwave, infrared also is reflected by metals (as is
most EMR into the ultraviolet). 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 (see thermal
radiation section below).
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 the
mechanism of vision involves the change in bonding of a single
molecule (retinal) which absorbs light in the rhodopsin in the retina
of the human eye.
Photosynthesis becomes possible in this range as
well, for similar reasons, as a single molecule of chlorophyll is
excited by a single photon. Animals that detect infrared make use of
small packets of water that change temperature, in an essentially
thermal process that involves many photons (see infrared sensing in
snakes). For this reason, infrared, microwaves and radio waves are
thought to damage molecules and biological tissue only by bulk
heating, not excitation from single photons of the radiation.
Visible light is able to affect a few molecules with single photons,
but usually not in a permanent or damaging way, in the absence of
power high enough to increase temperature to damaging levels. However,
in plant tissues that conduct photosynthesis, carotenoids act to
quench electronically excited chlorophyll produced by visible light in
a process called non-photochemical quenching, in order to prevent
reactions that would otherwise interfere with photosynthesis at high
light levels. Limited evidence indicate that some reactive oxygen
species are created by visible light in skin, and that these may have
some role in photoaging, in the same manner as ultraviolet A.
As frequency increases into the ultraviolet, photons now carry enough
energy (about three electron volts or more) to excite certain doubly
bonded molecules into permanent chemical rearrangement. In DNA, 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 (for UVB) skin
burns (sunburn) that are far worse than would be produced by simple
heating (temperature increase) effects. This property of causing
molecular damage that is out of proportion to heating effects, is
characteristic of all EMR with frequencies at the visible light range
and above. These properties of high-frequency EMR are due to quantum
effects that permanently damage materials and tissues at the molecular
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 called
photoionisation. The energy required for this is always larger than
about 10 electron volts (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).
X-rays and gamma rays
Electromagnetic radiation composed of photons that carry
minimum-ionization energy, or more, (which includes the entire
spectrum with shorter wavelengths), is therefore termed ionizing
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, 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.
Atmosphere and magnetosphere
Main articles: ozone layer, shortwave radio, skywave, and ionosphere
Rough plot of Earth's atmospheric absorption and scattering (or
opacity) of various wavelengths of electromagnetic radiation
Most UV and
X-rays are blocked by absorption first from molecular
nitrogen, and then (for wavelengths in the upper UV) from the
electronic excitation of dioxygen and finally ozone 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, as it is not energetic
enough to excite nitrogen, oxygen, or ozone, but too energetic to
excite molecular vibrational frequencies of water vapor.[citation
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.
Finally, at radio wavelengths longer than 10 meters or so (about
30 MHz), the air in the lower atmosphere remains transparent to
radio, but plasma in certain layers of the ionosphere begins to
interact with radio waves (see skywave). This property allows some
longer wavelengths (100 meters or 3 MHz) to be reflected and
results in shortwave radio beyond line-of-sight. However, certain
ionospheric effects begin to block incoming radiowaves from space,
when their frequency is less than about 10 MHz (wavelength longer
than about 30 meters).
Types and sources, classed by spectral band
Further information: Electromagnetic spectrum
Main article: Radio waves
Radio waves have the least amount of energy and the lowest frequency.
When radio waves impinge upon a conductor, they couple to the
conductor, travel along it and induce an electric current on the
conductor surface by moving the electrons of the conducting material
in correlated bunches of charge. Such effects can cover macroscopic
distances in conductors (such as radio antennas), since the wavelength
of radiowaves is long.
Main article: Microwaves
Microwaves are a form of electromagnetic radiation with wavelengths
ranging from as long as one meter to as short as one millimeter; with
frequencies between 300 MHz (0.3 GHz) and 300 GHz.
Main article: Infrared
This section is empty. You can help by adding to it. (October 2017)
Main article: Light
Natural sources produce EM radiation across the spectrum. EM radiation
with a wavelength between approximately 400 nm and 700 nm is
directly detected by the human 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.
Main article: Ultraviolet
This section is empty. You can help by adding to it. (October 2017)
Main article: X-rays
This section is empty. You can help by adding to it. (October 2017)
Main article: Gamma rays
This section is empty. You can help by adding to it. (October 2017)
Thermal radiation and electromagnetic radiation as a form of
Thermal radiation and Planck's law
The basic structure of matter 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 to thermal equilibrium and manifesting itself as
thermal energy, or even kinetic energy, in the material. With a few
exceptions related to high-energy photons (such as fluorescence,
harmonic generation, photochemical reactions, the photovoltaic 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 infrared lasers,
sufficiently intense visible and ultraviolet lasers can easily set
paper afire.
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 process makes ionizing
radiation far more dangerous per unit of energy than non-ionizing
radiation. 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
Infrared radiation in the spectral distribution of a black 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 the thermal energy temperature of) a
material, when it is absorbed.
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
The electromagnetic radiation in an opaque cavity at thermal
equilibrium is effectively a form of thermal energy, having maximum
Electromagnetic radiation and health
Electromagnetic radiation and health and Mobile phone
radiation and health
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 visible light) 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 only as it
affects 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.
Despite the commonly accepted results, some research has been
conducted to show that weaker non-thermal electromagnetic fields,
(including weak ELF magnetic fields, although the latter does not
strictly qualify as EM radiation), and modulated RF and
microwave fields have biological effects. Fundamental
mechanisms of the interaction between biological material and
electromagnetic fields at non-thermal levels are not fully
World Health Organization
World Health Organization has classified radio frequency
electromagnetic radiation as Group 2B - possibly carcinogenic.
This group contains possible carcinogens such as lead, DDT, and
styrene. For example, epidemiological studies looking for a
relationship between cell phone use and brain cancer development, have
been largely inconclusive, save to demonstrate that the effect, if it
exists, cannot be a large one.
At higher frequencies (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. All UV frequences have been classed as Group 1
carcinogens by the World Health Organization.
from sun exposure is the primary cause of skin cancer.
Thus, at UV frequencies and higher (and probably somewhat also in the
visible range), 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 as ionizing radiation due to the ability of
photons of this radiation to produce ions and free 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.
Derivation from electromagnetic theory
Main article: Electromagnetic wave equation
Electromagnetic waves were predicted by the classical laws of
electricity and magnetism, known as Maxwell's equations. Inspection of
Maxwell's equations without sources (charges or currents) results in
nontrivial solutions of changing electric and magnetic fields.
Maxwell's equations in free space:
displaystyle nabla cdot mathbf E =0
displaystyle nabla times mathbf E =- frac partial mathbf B
displaystyle nabla cdot mathbf B =0
displaystyle nabla times mathbf B =mu _ 0 epsilon _ 0 frac
partial mathbf E partial t
is a vector differential operator (see Del).
Cartesian coordinate system
Cartesian coordinate system
is defined in terms of partial derivative operators as
displaystyle nabla = hat i partial over partial x + hat j
partial over partial y + hat k partial over partial z
displaystyle mathbf E =mathbf B =mathbf 0 ,
For a more useful solution, we utilize vector identities, which work
for any vector, as follows:
displaystyle nabla times left(nabla times mathbf A right)=nabla
left(nabla cdot mathbf A right)-nabla ^ 2 mathbf A
The curl of equation (2):
displaystyle nabla times left(nabla times mathbf E right)=nabla
times left(- frac partial mathbf B partial t right)
Evaluating the left hand side:
displaystyle nabla times left(nabla times mathbf E right)=nabla
left(nabla cdot mathbf E right)-nabla ^ 2 mathbf E =-nabla ^ 2
simplifying the above by using equation (1).
Evaluating the right hand side:
displaystyle nabla times left(- frac partial mathbf B
partial t right)=- frac partial partial t left(nabla times mathbf
B right)=-mu _ 0 epsilon _ 0 frac partial ^ 2 mathbf E partial
Equations (6) and (7) are equal, so this results in a vector-valued
differential equation for the electric field, namely
displaystyle nabla ^ 2 mathbf E =mu _ 0 epsilon _ 0 frac
partial ^ 2 mathbf E partial t^ 2
Applying a similar pattern results in similar differential equation
for the magnetic field:
displaystyle nabla ^ 2 mathbf B =mu _ 0 epsilon _ 0 frac
partial ^ 2 mathbf B partial t^ 2 .
These differential equations are equivalent to the wave equation:
displaystyle nabla ^ 2 f= frac 1 c_ 0 ^ 2 frac partial ^
2 f partial t^ 2 ,
c0 is the speed of the wave in free space and
f describes a displacement
Or more simply:
displaystyle Box f=0
displaystyle Box =nabla ^ 2 - frac 1 c_ 0 ^ 2 frac
partial ^ 2 partial t^ 2 = frac partial ^ 2 partial x^ 2 +
frac partial ^ 2 partial y^ 2 + frac partial ^ 2 partial z^ 2
- frac 1 c_ 0 ^ 2 frac partial ^ 2 partial t^ 2
In the case of the electric and magnetic fields, the speed is:
displaystyle c_ 0 = frac 1 sqrt mu _ 0 epsilon _ 0
This is the speed of light in vacuum.
Maxwell's equations unified the
displaystyle epsilon _ 0
, the vacuum permeability
displaystyle mu _ 0
, and the speed of light itself, c0. This relationship had been
Wilhelm Eduard Weber
Wilhelm Eduard Weber and
Rudolf 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.
displaystyle mathbf E =mathbf E _ 0 fleft( hat mathbf k
cdot mathbf x -c_ 0 tright)
displaystyle mathbf E _ 0
is the constant amplitude,
is any second differentiable function,
displaystyle hat mathbf k
is a unit vector in the direction of propagation, and
displaystyle mathbf x
is a position vector.
displaystyle fleft( hat mathbf k cdot mathbf x -c_ 0
is a generic solution to the wave equation. In other words,
displaystyle nabla ^ 2 fleft( hat mathbf k cdot mathbf x
-c_ 0 tright)= frac 1 c_ 0 ^ 2 frac partial ^ 2 partial t^ 2
fleft( hat mathbf k cdot mathbf x -c_ 0 tright),
for a generic wave traveling in the
displaystyle hat mathbf k
This form will satisfy the wave equation.
displaystyle nabla cdot mathbf E = hat mathbf k cdot
mathbf E _ 0 f'left( hat mathbf k cdot mathbf x -c_ 0
displaystyle mathbf E cdot hat mathbf k =0
The first of
Maxwell's equations implies that the electric field is
orthogonal to the direction the wave propagates.
displaystyle nabla times mathbf E = hat mathbf k times
mathbf E _ 0 f'left( hat mathbf k cdot mathbf x -c_ 0
tright)=- frac partial mathbf B partial t
displaystyle mathbf B = frac 1 c_ 0 hat mathbf k
times mathbf E
The second of
Maxwell's equations yields the magnetic field. The
remaining equations will be satisfied by this choice of
displaystyle mathbf E ,mathbf B
The electric and magnetic field waves in the far-field travel at the
speed of light. They have a special restricted orientation and
displaystyle E_ 0 =c_ 0 B_ 0
, which can be seen immediately from the Poynting vector. The electric
field, magnetic field, and direction of wave propagation are all
orthogonal, and the wave propagates in the same direction as
displaystyle mathbf E times mathbf B
. Also, E and B far-fields in free space, which as wave solutions
depend primarily on these two Maxwell equations, are in-phase with
each other. This is guaranteed since the generic wave solution is
first order in both space and time, and the curl 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 same phase shift for both fields in each mathematical
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 as polarization. On a
quantum level, it is described as photon 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, 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.
Control of electromagnetic radiation
Electromagnetic radiation and health
Electromagnetic wave equation
Evanescent wave coupling
Finite-difference time-domain method
Impedance of free space
Near and far field
Risks and benefits of sun exposure
Sinusoidal plane-wave solutions of the electromagnetic wave equation
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Physics for the 21st Century Early Unification for Electromagnetism
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