Electromagnetic radiation is a fundamental
phenomenon of electromagnetism, behaving as
waves and also as particles called photons
which travel through space carrying radiant
energy. In a vacuum, it propagates at the
speed of light, normally in straight lines.
EMR is emitted and absorbed by charged particles.
As an electromagnetic wave, it has both electric
and magnetic field components, which synchronously
oscillate perpendicular to each other and
perpendicular to the direction of energy and
wave propagation.
In classical physics, EMR is produced when
charged particles are accelerated by forces
acting on them. Electrons are responsible
for emission of most EMR because they have
low mass, and therefore are easily accelerated
by a variety of mechanisms. Quantum processes
can also produce EMR, such as when atomic
nuclei undergo gamma decay, and processes
such as neutral pion decay.
EMR carries energy—sometimes called radiant
energy—through space continuously away from
the source. EMR also carries both momentum
and angular momentum. These properties may
all be imparted to matter with which it interacts.
When created, EMR is produced from other types
of energy and it is converted to other types
of energy when it is destroyed.
The electromagnetic spectrum, in order of
increasing frequency and decreasing wavelength,
can be divided, for practical engineering
purposes, into radio waves, microwaves, infrared
radiation, visible light, ultraviolet radiation,
X-rays and gamma rays. The eyes of various
organisms sense a relatively small range of
frequencies of EMR near and including the
visible spectrum or light. Visible light is
that part of the spectrum to which human eyes
respond Higher frequencies have more energy
in the photons, according to the well-known
law E=hν, where E is the energy per photon,
ν is the frequency carried by the photon,
and h is Planck's constant. A single gamma
ray photon carries far more energy than a
single photon of visible light.
The photon is the quantum of the electromagnetic
interaction, and is the basic constituent
of all forms of EMR. The quantum nature of
light becomes more apparent at high frequencies.
Such photons behave more like particles than
lower-frequency photons do.
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 distance
from the source. Both types of waves can have
a waveform which is an arbitrary time function.
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.
Electromagnetic radiation is associated with
EM fields that are free to propagate themselves
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, as for example
with simple magnets and static electricity
phenomena. In EMR, the magnetic and electric
fields are each induced by changes in the
other type of field, thus propagating itself
as a wave. This close relationship assures
that both types of fields in EMR stand in
phase and in a fixed ratio of intensity to
each other, with maxima and nodes in each
found at the same places in space.
The effects of EMR upon biological systems
depend both upon the radiation's power and
frequency. For lower frequencies of EMR up
to those of visible light, the damage done
to cells and also to many ordinary materials
under such conditions is determined mainly
by heating effects, and thus by the radiation
power. By contrast, for higher frequency radiations
at ultraviolet frequencies and above the damage
to chemical materials and living cells by
EMR is far larger than that done by simple
heating, due to the ability of single photons
in such high frequency EMR to damage individual
molecules chemically.
Physics
Theory
Maxwell’s equations for EM fields far from
sources
James Clerk Maxwell first formally postulated
electromagnetic waves. These were subsequently
confirmed by Heinrich Hertz. 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.
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 for it.
Near and far fields
Maxwell's equations established that some
charges and currents produce a local type
of electromagnetic field near them that does
not have the behavior of EMR. In particular,
according to Maxwell, currents directly produce
a magnetic field, but it is of a magnetic
dipole type which dies out rapidly with distance
from the current. In a similar manner, moving
charges being separated from each other in
a conductor by a changing electrical potential
produce an electric dipole type electrical
field, but this also dies away very quickly
with distance. Both of these fields make up
the near-field near the EMR source. Neither
of these behaviors are responsible for EM
radiation. Instead, they cause electromagnetic
field behavior that only efficiently transfers
power to a receiver very close to the source,
such as the magnetic induction inside a transformer,
or the feedback behavior 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”
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
back and forth, 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 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". The far-fields propagate without
ability for the transmitter to affect them,
and 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.
Because such waves conserve the amount of
energy they transmit through any spherical
boundary surface drawn around their source,
and because such surfaces have an area that
is defined by the square of the distance from
the source, the power of EM radiation always
varies according to an inverse-square law.
This is in contrast to dipole parts of the
EM field close to the source, 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
dies away rapidly with distance, with its
energy either rapidly returning to the transmitter,
or else absorbed by a nearby receiver.
The far-field 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 of these processes for producing electric
and magnetic EMR fields have a different dependence
on distance than do near-field dipole electric
and magnetic fields, and 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
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.
In the Liénard–Wiechert potential formulation
of the electric and magnetic fields due to
motion of a single particle, 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 seen
to be associated with the electromagnetic
near-field, and do not comprise EM radiation.
Properties
The physics of electromagnetic radiation is
electrodynamics. 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.
Since light is an oscillation it is not affected
by travelling 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 effect.
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 disperses into a visible
spectrum passing through a prism, because
of the wavelength dependent refractive index
of the prism material; that is, each component
wave within the composite light is bent a
different amount.
EM radiation exhibits both wave properties
and particle properties at the same time.
Both wave and particle characteristics have
been confirmed in a large number of 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 of light. 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
problem.
There are experiments in which the wave and
particle natures of electromagnetic waves
appear in the same experiment, such as the
self-interference of a single photon. True
single-photon experiments can be done today
in undergraduate-level labs. 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 once.
A quantum theory of the interaction between
electromagnetic radiation and matter such
as electrons is described by the theory of
quantum electrodynamics.
Wave model
Electromagnetic radiation is a transverse
wave, meaning that the oscillations of the
waves 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.
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.
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.
An important aspect of the nature of light
is 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 second. Light
usually has a spectrum of 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 nuclei. Frequency is inversely proportional
to wavelength, according to the equation:
where v is the speed of the wave, f is the
frequency and λ is the wavelength. As waves
cross boundaries between different media,
their speeds change but their frequencies
remain constant.
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 or
as it is more commonly known as, radio-frequency
interference.
The energy in electromagnetic waves is sometimes
called radiant energy.
Particle model and quantum theory
An anomaly arose in the late 19th century
involving a contradiction between the wave
theory of light on the one hand, and on the
other, observers' actual 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 only as discrete bundles
or packets of energy. These packets were called
quanta. Later, Albert Einstein proposed that
the quanta of light might be regarded as real
particles, and 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, by
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 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:
The source of Einstein's proposal that light
was composed of particles 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. When an electron in an excited
molecule or atom descends to a lower energy
level, it emits a photon of light equal 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. When the emission
of the photon is immediate, this phenomenon
is called fluorescence, a type of photoluminescence.
An example is visible light emitted from fluorescent
paints, in response to ultraviolet. Many other
fluorescent emissions are known in spectral
bands other than visible light. When the emission
of the photon is delayed, the phenomenon is
called phosphorescence.
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
if an object has a large mass, and it was
not until a bold proposition by Louis de Broglie
in 1924 that the scientific community realised
that electrons also exhibited wave–particle
duality.
Wave and particle effects of electromagnetic
radiation
Together, wave and particle effects explain
the emission and absorption spectra of EM
radiation, wherever it is seen. 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, absorbing certain
frequencies of the light between emitter and
detector/eye, then emitting them in all directions,
so that 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 the
emitting gas is glowing 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.
Today, scientists use these phenomena to perform
various chemical determinations for the composition
of gases lit from behind and for glowing gases.
Spectroscopy determines what chemical elements
a star is composed of. Spectroscopy is also
used in the determination of the distance
of a star, using the red shift.
Speed of propagation
Any electric charge that accelerates, or any
changing magnetic field, produces electromagnetic
radiation. Electromagnetic information about
the charge travels at the speed of light.
Accurate treatment thus incorporates a concept
known as retarded time, which adds to the
expressions for the electrodynamic electric
field and magnetic field. These extra terms
are responsible for electromagnetic radiation.
When any wire conducts alternating current,
electromagnetic radiation is propagated at
the same frequency as the electric 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 motion.
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 transition state which 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 in atoms when photons are
radiated as the atom shifts from one stationary
state to another.
Depending on the circumstances, electromagnetic
radiation may behave as a wave or as particles.
As a wave, it is characterized by a velocity,
wavelength, and frequency. When considered
as particles, they are known as photons, and
each has an energy related to the frequency
of the wave given by Planck's relation E = hν,
where E is the energy of the photon, h = 6.626
× 10−34 J·s is Planck's constant, and
ν is the frequency of the wave.
One rule is always obeyed regardless of the
circumstances: EM radiation in a vacuum always
travels at the speed of light, relative to
the observer, regardless of the observer's
velocity.
In a medium, 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.
Special theory of relativity
By the late nineteenth century, however, a
handful of experimental anomalies remained
that 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, or else
otherwise it would depend on the speed of
observer relative to the "medium" which supposedly
"carried" the electromagnetic wave. Experiments
failed to find any observer effect, however.
In 1905, Albert Einstein proposed that space
and time appeared to be velocity-changeable
entities, not only for light propagation,
but all other processes and laws as well.
These changes then automatically 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
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 William
Herschel, the astronomer. Herschel 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 infrared.
In 1801, the German physicist Johann Wilhelm
Ritter made the discovery of 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 were capable of causing
chemical reactions.
In 1862-4 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 all consisted of propagating
disturbances in the electromagnetic field.
Radio waves were not first detected from a
natural source, but were rather produced deliberately
and artificially by the German scientist 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 equations. Hertz also
developed ways to detect these waves, and
produced and characterized what were later
termed radio waves and microwaves.
Wilhelm Röntgen discovered and named X-rays.
After experimenting with high voltages applied
to an evaccuated tube on 8 November 1895,
he noticed a fluorescence on a nearby plate
of coated glass. In one month, he discovered
the main properties of X-rays that we understand
to this day.
The last portion of the EM spectrum was discovered
associated with radioactivity. 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 and beta rays by 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 demonstrated
that gamma rays are electromagnetic radiation,
not particles, and in 1914 Rutherford and
Edward Andrade measured their wavelengths,
and found that they were similar to X-rays
but with shorter wavelengths and higher frequency.
Electromagnetic spectrum
In general, EM radiation is classified by
wavelength into radio, microwave, infrared,
the visible spectrum we perceive as visible
light, ultraviolet, X-rays, and gamma rays.
Arbitrary electromagnetic waves can always
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
vacuum.
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 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 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.
Radio and microwave heating and currents,
and infrared heating
When EM radiation interacts with matter, its
behavior changes qualitatively as its frequency
changes. 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 results in absorption of the EMR,
or else separations of charges that cause
generation of new 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. Infrared EMR interacts with
dipoles present in single molecules, which
change as atoms vibrate at the ends of a single
chemical bond. For this reason, infrared is
reflected by metals but is absorbed by a wide
range of substances, causing them to increase
in temperature as the vibrations dissipate
as heat. In the same process, bulk substances
radiate in the infrared spontaneously.
Reversible and nonreversible molecular changes
from visible light
As frequency increases into the visible range,
photons of EMR 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 which absorbs light in the
rhodopsin 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 which
detect infrared do not use such single molecule
processes, but are forced to make use of small
packets of water which change temperature,
in an essentially thermal process that involves
many photons. 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 carry on photosynthesis, carotenoids
act to quench electronically excited chlorophyll
produced by visible light in a process called
non-photochemical quenching, in order to prevent
reactions which would otherwise interfere
with photosynthesis at high light levels.
There is also some limited evidence 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 does.
Molecular damage from ultraviolet
As a photon interacts with single atoms and
molecules, the effect depends on the amount
of energy the photon carries. As frequency
increases beyond visible into the ultraviolet,
photons now carry enough energy to excite
certain doubly bonded molecules into permanent
chemical rearrangement. If these molecules
are biological molecules in DNA, this causes
lasting damage. DNA is also indirectly damaged
by reactive oxygen species produced by ultraviolet
A, 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 skin burns which are far worse
than would be produced by simple heating effects.
This property of causing molecular damage
that is far out of proportion to all temperature-changing
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 which cause
permanent damage to materials and tissues
at the single molecular level.
Ionization and extreme types of molecular
damage from X-rays and gamma rays
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 corresponding with wavelengths smaller
than 124 nm. This high end of the ultraviolet
spectrum with energies in the approximate
ionization range, is sometimes called "extreme
UV.".
Electromagnetic radiation composed of photons
that carry minimum-ionization energy, or more,,
is therefore termed ionizing radiation.. 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 are ionizing radiation.
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 from
the skin, since the higher end of the X-ray
spectrum, and all of the gamma ray spectrum,
are penetrating to matter. It is this type
of damage which causes these types of radiation
to be especially carefully monitored, due
to their hazard, even at comparatively low-energies,
to all living organisms.
Propagation and absorption in the Earth's
atmosphere and magnetosphere
Most electromagnetic waves of higher frequency
than visible light are blocked by absorption
first from the magnetosphere and then from
the electronic excitation in ozone and dioxygen,
and by ionization of air for energies in the
extreme UV and above. Visible light is well
transmitted in air, as it is not energetic
enough to excite oxygen, but too energetic
to excite molecular vibrational frequencies
of molecules in air.
Below visible light, a number of 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, the air in the lower atmosphere
remains transparent to radio, but plasma in
certain layers of the ionosphere of upper
Earth atmosphere begins to interact with radio
waves. This property allows some longer wavelengths
to be reflected and results in farther shortwave
radio than can be obtained by line-of-sight.
However, certain ionospheric effects begin
to block incoming radiowaves from space, when
their frequency is less than about 10 MHz.
Types and sources, classed by spectral band
See electromagnetic spectrum for detail
Radio waves
When EM radiation at the frequencies for which
it is referred to as "radio waves" impinges
upon a conductor, it couples to the conductor,
travels along it, and induces an electric
current on the surface of the conductor by
moving the electrons of the conducting material
in correlated bunches of charge. Such effects
can cover macroscopic distances in conductors,
since the wavelength of radiowaves is long,
by human scales. Radio waves thus have the
most overtly "wave-like" characteristics of
all the types of EMR, since their waves are
so long.
Microwaves
Infrared
Visible 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 and ultraviolet are also sometimes
referred to as light, especially when visibility
to humans is not relevant.
Ultraviolet
X-rays
Gamma rays
Thermal radiation and electromagnetic radiation
as a form of heat
The basic structure of matter involves charged
particles bound together in many different
ways. When electromagnetic radiation is incident
on matter, it causes the charged particles
to oscillate and gain energy. The ultimate
fate of this energy depends on the situation.
It could be immediately re-radiated and appear
as scattered, reflected, or transmitted radiation.
It may also get dissipated into other microscopic
motions within the matter, coming to thermal
equilibrium and manifesting itself as thermal
energy in the material. With a few exceptions
related to high-energy photons, absorbed electromagnetic
radiation simply deposits its energy by heating
the material. This happens both 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 also easily set paper afire.
Ionizing electromagnetic radiation creates
high-speed electrons in a material and breaks
chemical bonds, but after these electrons
collide many times with other atoms in the
material eventually most of the energy is
downgraded to thermal energy; this whole process
happens 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 the
ultraviolet spectrum, 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 produce.
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, the word
"heat" is a highly 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" a material, when it is
absorbed.
The inverse or time-reversed process of absorption
is responsible for 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.
Thermal radiation is an important mechanism
of heat transfer.
The electromagnetic radiation in an opaque
cavity at thermal equilibrium is effectively
a form of thermal energy, having maximum radiation
entropy.
Biological effects
The effects of electromagnetic radiation upon
living cells, including those in humans, depends
upon the power and the frequency of the radiation.
For low-frequency radiation the best-understood
effects are those due to radiation power alone,
acting through the effect of simple heating
when the radiation is absorbed by the cell.
For these thermal effects, the frequency of
the radiation is important only as it affects
radiation penetration into the organism. Initially,
it was believed that low frequency fields
that were too weak to cause significant heating
could not possibly have any biological effect.
Despite this opinion among researchers, evidence
has accumulated that supports the existence
of complex biological effects of weaker non-thermal
electromagnetic fields,, and modulated RF
and microwave fields. Fundamental mechanisms
of the interaction between biological material
and electromagnetic fields at non-thermal
levels are not fully understood. Bioelectromagnetics
is the study of these interactions and effects.
The World Health Organization has classified
radiofrequency electromagnetic radiation as
a possible group 2b carcinogen. This group
contains possible carcinogens with weaker
evidence, at the same level as coffee and
automobile exhaust. For example, there have
been a number of epidemiological studies of
looking for a relationship between cell phone
use and brain cancer development, which have
been largely inconclusive, save to demonstrate
that the effect, if it exists, cannot be a
large one. See the main article referenced
above.
At higher frequencies, the effects of individual
photons of the radiation begin to become important,
as these now have enough energy individually
directly or indirectly to damage biological
molecules. All frequences of UV radiation
have been classed as Group 1 carcinogens by
the World Health Organization. Ultraviolet
radiation from sun exposure is the primary
cause of skin cancer.
Thus, at UV frequencies and higher, electromagnetic
radiation does far more damage to biological
systems than simple heating predicts. This
is most obvious in the "far" ultraviolet,
and also 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. Since such
radiation can produce severe damage to life
at powers that produce very little heating,
it is considered far more dangerous than the
rest of the electromagnetic spectrum.
Derivation from electromagnetic theory
Electromagnetic waves as a general phenomenon
were predicted by the classical laws of electricity
and magnetism, known as Maxwell's equations.
Inspection of Maxwell's equations without
sources results in, along with the possibility
of nothing happening, nontrivial solutions
of changing electric and magnetic fields.
Beginning with Maxwell's equations in free
space:
where
is a vector differential operator.
One solution,
is trivial.
For a more useful solution, we utilize vector
identities, which work for any vector, as
follows:
To see how we can use this, take the curl
of equation:
Evaluating the left hand side:
where we simplified the above by using equation.
Evaluate the right hand side:
Equations and are equal, so this results in
a vector-valued differential equation for
the electric field, namely
Applying a similar pattern results in similar
differential equation for the magnetic field:
These differential equations are equivalent
to the wave equation:
where
c0 is the speed of the wave in free space
and
f describes a displacement
Or more simply:
where is d'Alembertian:
Notice that, in the case of the electric and
magnetic fields, the speed is:
This is the speed of light in vacuum. Maxwell's
equations unified the vacuum permittivity
, the vacuum permeability , and the speed
of light itself, c0. This relationship had
been discovered by 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.
But these are only two equations and we started
with four, so there is still more information
pertaining to these waves hidden within Maxwell's
equations. Let's consider a generic vector
wave for the electric field.
Here, is the constant amplitude, is any second
differentiable function, is a unit vector
in the direction of propagation, and is a
position vector. We observe that is a generic
solution to the wave equation. In other words
for a generic wave traveling in the direction.
This form will satisfy the wave equation,
but will it satisfy all of Maxwell's equations,
and with what corresponding magnetic field?
The first of Maxwell's equations implies that
electric field is orthogonal to the direction
the wave propagates.
The second of Maxwell's equations yields the
magnetic field. The remaining equations will
be satisfied by this choice of .
Not only are the electric and magnetic field
waves in the far-field traveling at the speed
of light, but they always have a special restricted
orientation and proportional magnitudes, , 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 . Also, E and B far-fields
in free space, which as wave solutions depend
primarily on these two Maxwell equations,
are always 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 spacial 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 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; but 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. A great 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.
See also
References
Further reading
Hecht, Eugene. Optics. Pearson Education.
ISBN 0-8053-8566-5. 
Serway, Raymond A.; Jewett, John W.. Physics
for Scientists and Engineers. Brooks Cole.
ISBN 0-534-40842-7. 
Tipler, Paul. Physics for Scientists and Engineers:
Electricity, Magnetism, Light, and Elementary
Modern Physics. W. H. Freeman. ISBN 0-7167-0810-8. 
Reitz, John; Milford, Frederick; Christy,
Robert. Foundations of Electromagnetic Theory.
Addison Wesley. ISBN 0-201-52624-7. 
Jackson, John David. Classical Electrodynamics.
John Wiley & Sons. ISBN 0-471-30932-X. 
Allen Taflove and Susan C. Hagness. Computational
Electrodynamics: The Finite-Difference Time-Domain
Method, 3rd ed. Artech House Publishers. ISBN 1-58053-832-0. 
External links
Electromagnetism – a chapter from an online
textbook
Electromagnetic Radiation – an introduction
for electrical engineers
Electromagnetic Waves from Maxwell's Equations
on Project PHYSNET.
Radiation of atoms? e-m wave, Polarisation,
...
An Introduction to The Wigner Distribution
in Geometric Optics
The windows of the electromagnetic spectrum,
on Astronoo
Introduction to light and electromagnetic
radiation course video from the Khan Academy
Lectures on electromagnetic waves course video
and notes from MIT Professor Walter Lewin
