Dictionary Definition
radiate
Verb
1 send out rays or waves; "The sun radiates
heat"
2 send out real or metaphoric rays; "She radiates
happiness"
3 extend or spread outward from a center or focus
or inward towards a center; "spokes radiate from the hub of the
wheel"; "This plants radiates spines in all directions" [syn:
ray]
4 especially of the complexion: show a strong
bright color, such as red or pink; "Her face glowed when she came
out of the sauna" [syn: glow, beam, shine]
5 cause to be seen by emitting light as if in
rays; "The sun is radiating"
6 experience a feeling of wellbeing or
happiness, as from good health or an intense emotion; "She was
beaming with joy"; "Her face radiated with happiness" [syn:
glow, beam, shine]
7 issue or emerge in rays or waves; "Heat
radiated from the metal box"
8 spread into new habitats and produce variety or
variegate; "The plants on this island diversified" [syn: diversify]
User Contributed Dictionary
English
Verb
 To extend, send or spread out from a center like radii.
 To emit rays or waves.
 The stove radiates heat.
 To come out or proceed in rays or waves.
 The heat radiates from a stove.
 To illuminate.
 To expose to ionizing radiation, such as by radiography.
 To manifest oneself in a glowing manner.
 In the context of "ecology, intransitive": to spread into new habitats, migrate.
Synonyms
 (to expose to radiation): irradiate
Derived terms
Translations
to extend from a center
 Finnish: säteillä
to emit rays or waves
 Finnish: säteillä
to come out or proceed in rays
 Finnish: säteillä
to illuminate
 Finnish: valaista
to expose to ionizing radiation
 Finnish: säteilyttää, altistaa säteilylle
to manifest in a glowing manner
 Finnish: säteillä
to spread into new habitats
 Finnish: levitä, levittäytyä
translations to be checked
Adjective
 radiating from a center.
 surrounded by rays, such as the head of a saint in a religious picture.
 having parts radiating from the center, such as the petals in many flowers.
 having radial symmetry, such as a seastar.
Translations
radiating from a center
 Finnish: säteilevä
surrounded by rays
 Finnish: säteilevä
having parts radiating from the center
 Finnish: säteittäinen
having by radial symmetry
 Finnish: säteittäinen
Italian
Verb form
radiateExtensive Definition
Electromagnetic (EM) radiation is a selfpropagating
wave in space or through
transparent matter.
EM radiation has an electric
and magnetic
field component which oscillate in phase
perpendicular to each other and to the direction of energy propagation.
Electromagnetic radiation is classified into types according to the
frequency of the wave,
these types include (in order of increasing frequency): radio waves,
microwaves, terahertz
radiation, infrared
radiation, visible
light, ultraviolet
radiation, Xrays and gamma rays. Of
these, radio waves have the longest wavelengths and Gamma rays have
the shortest. A small window of frequencies, called visible
spectrum or light, is
sensed by the eye of
various organisms, with
variations of the limits of this narrow spectrum. Light is
sometimes used in a broader sense, referring to EM radiation.
Physics
Theory
Electromagnetic waves were first postulated by James Clerk Maxwell and subsequently confirmed by Heinrich Hertz. Maxwell derived a wave form of the electric and magnetic equations, revealing the wavelike 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 timevarying electric
field generates a magnetic
field and vice versa. Therefore, as an oscillating electric
field generates an oscillating magnetic field, the magnetic field
in turn generates an oscillating electric field, and so on. These
oscillating fields together form an electromagnetic wave.
A quantum
theory of the interaction between electromagnetic radiation and
matter such as electrons is described by the theory of quantum
electrodynamics.
Properties
Electric and magnetic fields obey the properties
of superposition,
so fields due to particular particles or timevarying electric or
magnetic fields contribute to the fields due to other causes. (As
these fields are vector fields, all magnetic and electric field
vectors add together according to vector
addition.) These properties cause various phenomena including
refraction and
diffraction. For
instance, a travelling EM wave incident on an atomic structure
induces oscillation in the atoms, thereby causing them to emit
their own EM waves. These
emissions then alter the impinging wave through
interference.
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. In nonlinear media such as some
crystals, however,
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 disperses into a visible spectrum
as light is shone through a prism because of refraction.
EM radiation exhibits both wave properties and
particle
properties at the same time (see waveparticle
duality). The wave characteristics are more apparent when EM
radiation is measured over relatively large timescales and over
large distances, and the particle characteristics are more evident
when measuring small distances and timescales. Both characteristics
have been confirmed in a large number of experiments.
There are experiments in which the wave and
particle natures of electromagnetic waves appear in the same
experiment, such as the diffraction of a single photon. When a single photon is
sent through two slits, it passes through both of them interfering
with itself, as waves do, yet is detected by a photomultiplier or other
sensitive detector only once. Similar selfinterference is observed
when a single photon is sent into a Michelson
interferometer or other interferometers.
Wave model
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 which sum together to form the resultant wave. Different frequencies undergo different angles of refraction. 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: \displaystyle v=f\lambda
Particle model
Because energy of an EM wave is quantized, in the particle model of EM radiation, a wave consists of discrete packets of energy, or quanta, called photons. The frequency of the wave is proportional to the magnitude of the particle's energy. Moreover, because photons are emitted and absorbed by charged particles, they act as transporters of energy. The energy per photon can be calculated by Planck's equation: \displaystyle E=hf
where E is the energy, h is Planck's
constant, and f is frequency. This photonenergy expression is
a particular case of the energy levels of the more general
electromagnetic oscillator whose average energy, which is used to
obtain Planck's radiation law, can be shown to differ sharply from
that predicted by the equipartition
principle at low temperature, thereby establishes a failure of
equipartition due to quantum effects at low temperature.
As a photon is absorbed by an atom, it excites an electron, elevating it to a
higher energy
level. If the energy is great enough, so that the electron
jumps to a high enough energy level, it may escape the positive
pull of the nucleus and be liberated from the atom in a process
called photoionisation.
Conversely, an electron that descends to a lower energy level in an
atom emits a photon of light equal to the energy difference. Since
the energy levels of electrons in atoms are discrete, each element
emits and absorbs its own characteristic frequencies.
Together, these effects explain the absorption
spectra of light. The dark
bands in the spectrum are due to the atoms in the intervening
medium absorbing different frequencies of the light. The
composition of the medium through which the light travels
determines the nature of the absorption spectrum. For instance,
dark bands in the light emitted by a distant star are due to the atoms in the
star's atmosphere. These bands correspond to the allowed energy
levels in the atoms. A similar phenomenon occurs for
emission. As the electrons descend to lower energy levels, a
spectrum is emitted that represents the jumps between the energy
levels of the electrons. This is manifested in the
emission spectrum of nebulae. Today, scientists use
this phenomenon to observe what elements a certain star is composed
of. It is also used in the determination of the distance of a star,
using the socalled red
shift.
Speed of propagation
Any electric charge which 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 (as opposed to advanced time, which is unphysical in light of causality), which adds to the expressions for the electrodynamic electric field and magnetic field. These extra terms are responsible for electromagnetic radiation. When any wire (or other conducting object such as an antenna) conducts alternating current, electromagnetic radiation is propagated at the same frequency as the electric current. Depending on the circumstances, it may behave as a wave or as particles. As a wave, it is characterized by a velocity (the speed of light), 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 × 1034 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. (This observation led to Albert
Einstein's development of the theory of special
relativity.)
In a medium (other than vacuum), velocity
of propagation 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.
Electromagnetic spectrum
NUV = Near ultraviolet Visible
light NIR = Near infrared MIR = Moderate
infrared FIR = Far infrared
Radio waves:
EHF = Extremely
high frequency (Microwaves) SHF = Super
high frequency (Microwaves) UHF = Ultrahigh
frequency VHF = Very
high frequency HF = High
frequency MF = Medium
frequency LF = Low
frequency VLF = Very low
frequency VF = Voice
frequency ELF = Extremely
low frequency]]
Generally, EM radiation is classified by
wavelength into electrical
energy, radio,
microwave, infrared, the visible
region we perceive as light, ultraviolet, Xrays and gamma
rays.
The behavior of EM radiation depends on its
wavelength. Higher frequencies have shorter wavelengths, and lower
frequencies have longer wavelengths. When EM radiation interacts
with single atoms and molecules, its behavior depends on the amount
of energy per quantum it carries. Electromagnetic radiation can be
divided into octaves — as
sound waves are — winding up with eightyone octaves.
Spectroscopy
can detect a much wider region of the EM spectrum than the visible
range of 400 nm to 700 nm. A common laboratory spectroscope can
detect wavelengths from 2 nm to 2500 nm. Detailed information about
the physical properties of objects, gases, or even stars can be
obtained from this type of device. It is widely used in astrophysics. For example,
hydrogen atoms
emit radio waves of
wavelength 21.12
cm.
Light
EM radiation with a wavelength between
approximately 400 nm
and 700 nm is 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,
especially when the visibility to humans is not relevant.
If radiation having a frequency in the visible
region of the EM spectrum reflects off of an object, say, a bowl of
fruit, and then strikes our eyes, this results in our visual
perception of the scene. Our brain's visual system processes
the multitude of reflected frequencies into different shades and
hues, and through this notentirelyunderstood psychophysical
phenomenon, most people perceive a bowl of fruit.
At most wavelengths, however, the information
carried by electromagnetic radiation is not directly detected by
human senses. Natural sources produce EM radiation across the
spectrum, and our technology can also manipulate a broad range of
wavelengths. Optical
fiber transmits light which, although not suitable for direct
viewing, can carry data that can be translated into sound or an
image. The coding used in such data is similar to that used with
radio waves.
Radio waves
Radio waves can be made to carry information by
varying a combination of the amplitude, frequency and phase of the
wave within a frequency band.
When EM radiation impinges upon a conductor,
it couples to the conductor, travels along it, and induces
an electric current on the surface of that conductor by exciting
the electrons of the conducting material. This effect (the skin effect)
is used in antennas. EM radiation may also cause certain molecules
to absorb energy and thus to heat up; this is exploited in microwave
ovens.
Derivation
Electromagnetic waves as a general phenomenon were predicted by the classical laws of electricity and magnetism, known as Maxwell's equations. If you inspect Maxwell's equations without sources (charges or currents) then you will find that, along with the possibility of nothing happening, the theory will also admit nontrivial solutions of changing electric and magnetic fields. Beginning with Maxwell's equations for free space:
 \nabla \cdot \mathbf = 0 \qquad \qquad \qquad \ \ (1)
 \nabla \times \mathbf = \frac \mathbf \qquad \qquad (2)
 \nabla \cdot \mathbf = 0 \qquad \qquad \qquad \ \ (3)
 \nabla \times \mathbf = \mu_0 \epsilon_0 \frac \mathbf \qquad \ \ \ (4)
 where

 \nabla is a vector differential operator (see Del).
One solution,

 \mathbf=\mathbf=\mathbf,
To see the more interesting one, we utilize
vector identities, which work for any vector, as follows:

 \nabla \times \left( \nabla \times \mathbf \right) = \nabla \left( \nabla \cdot \mathbf \right)  \nabla^2 \mathbf
To see how we can use this take the curl of
equation (2):

 \nabla \times \left(\nabla \times \mathbf \right) = \nabla \times \left(\frac \right) \qquad \qquad \qquad \quad \ \ \ (5) \,
Evaluating the left hand side:

 \nabla \times \left(\nabla \times \mathbf \right) = \nabla\left(\nabla \cdot \mathbf \right)  \nabla^2 \mathbf =  \nabla^2 \mathbf \qquad \quad \ (6) \,
 where we simplified the above by using equation (1).
Evaluate the right hand side:

 \nabla \times \left(\frac \right) = \frac \left( \nabla \times \mathbf \right) = \mu_0 \epsilon_0 \frac \mathbf \qquad (7)
Equations (6) and (7) are equal, so this results
in a vectorvalued 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:

 \nabla^2 f = \frac \frac \,
 where

 c0 is the speed of the wave in free space and
 f describes a displacement
Or more simply:

 \Box^2 f = 0
 where \Box^2 is d'Alembertian:

 \Box^2 = \nabla^2  \frac \frac = \frac + \frac + \frac  \frac \frac \
Notice that in the case of the electric and
magnetic fields, the speed is:

 c_0 = \frac
Which, as it turns out, is the speed of
light in free space. Maxwell's equations have unified the
permittivity of free space \epsilon_0, the permeability of free
space \mu_0, and the speed of light itself, c0. Before this
derivation it was not known that there was such a strong
relationship between light and electricity and magnetism.
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.
 \mathbf = \mathbf_0 f\left( \hat \cdot \mathbf  c_0 t \right)
Here \mathbf_0 is the constant amplitude, f is
any second differentiable function, \hat is a unit vector in the
direction of propagation, and is a position vector. We observe that
f\left( \hat \cdot \mathbf  c_0 t \right) is a generic solution to
the wave equation. In other words
 \nabla^2 f\left( \hat \cdot \mathbf  c_0 t \right) = \frac \frac f\left( \hat \cdot \mathbf  c_0 t \right),
This form will satisfy the wave equation, but
will it satisfy all of Maxwell's equations, and with what
corresponding magnetic field?
 \nabla \cdot \mathbf = \hat \cdot \mathbf_0 f'\left( \hat \cdot \mathbf  c_0 t \right) = 0
 \mathbf \cdot \hat = 0
The first of Maxwell's equations implies that
electric field is orthogonal to the direction the wave
propagates.
 \nabla \times \mathbf = \hat \times \mathbf_0 f'\left( \hat \cdot \mathbf  c_0 t \right) = \frac \mathbf
 \mathbf = \frac \hat \times \mathbf
The second of Maxwell's equations yields the
magnetic field. The remaining equations will be satisfied by this
choice of \mathbf,\mathbf.
Not only are the electric and magnetic field
waves traveling at the speed of light, but they have a special
restricted orientation and proportional magnitudes, 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 \mathbf \times \mathbf.
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.
See also
 Control of electromagnetic radiation
 Electromagnetic pulse
 Electromagnetic spectrum
 Bioelectromagnetism
 Electromagnetic radiation and health
 Electromagnetic wave equation
 Finitedifference timedomain method
 Helicon
 Klystron
 Light
 Maxwell's equations
 Photon polarization
 Radiant energy
 Sinusoidal planewave solutions of the electromagnetic wave equation
 Radiation reaction
References
 Computational Electrodynamics: The FiniteDifference TimeDomain Method, 3rd ed. ">http://www.artechhouse.com/default.asp?Frame=Book.asp&Book=1580538320&Country=US&Continent=NO&State=}}
External links
 Electromagnetism  a chapter from an online textbook
 Electromagnetic Waves from Maxwell's Equations on Project PHYSNET.
 Conversion of frequency to wavelength and back  electromagnetic, radio and sound waves
 eBooks on Electromagnetic radiation and RF
 The Science of Spectroscopy  supported by NASA. Spectroscopy education wiki and films  introduction to light, its uses in NASA, space science, astronomy, medicine & health, environmental research, and consumer products.
radiate in Arabic: موجة كهرومغناطيسية
radiate in Min Nan: Tiānchûpho
radiate in Bosnian: Elektromagnetno
zračenje
radiate in Bulgarian: Електромагнитно
излъчване
radiate in Catalan: Radiació
electromagnètica
radiate in Czech: Elektromagnetické záření
radiate in Danish: Elektromagnetisk
stråling
radiate in German: Elektromagnetische
Welle
radiate in Estonian: Elektromagnetlaine
radiate in Modern Greek (1453):
Ηλεκτρομαγνητική ακτινοβολία
radiate in Spanish: Radiación
electromagnética
radiate in Esperanto: Elektromagneta ondo
radiate in Basque: Erradiazio
elektromagnetiko
radiate in French: Rayonnement
électromagnétique
radiate in Galician: Radiación
electromagnética
radiate in Gujarati: વિદ્યુતચુંબકીય તરંગો
radiate in Korean: 전자기파
radiate in Hindi: विद्युतचुंबकीय विकिरण
radiate in Croatian: Elektromagnetsko
zračenje
radiate in Indonesian: Radiasi
elektromagnetik
radiate in Icelandic: Rafsegulgeislun
radiate in Italian: Radiazione
elettromagnetica
radiate in Hebrew: קרינה אלקטרומגנטית
radiate in Lithuanian: Elektromagnetinė
banga
radiate in Limburgan: Elektromagnetische
straoling
radiate in Lojban: dicmakseldi'e
radiate in Hungarian: Elektromágneses
hullám
radiate in Malay (macrolanguage): Sinaran
elektromagnet
radiate in Dutch: Elektromagnetische
straling
radiate in Japanese: 電磁波
radiate in Norwegian: Elektromagnetisk
stråling
radiate in Norwegian Nynorsk: Elektromagnetisk
stråling
radiate in Oromo: Electromagnetic
radiation
radiate in Polish: Promieniowanie
elektromagnetyczne
radiate in Portuguese: Radiação
electromagnética
radiate in Romanian: Radiaţie
electromagnetică
radiate in Quechua: Iliktrumaqnitiku
illanchay
radiate in Russian: Электромагнитное
излучение
radiate in Simple English: Electromagnetic
radiation
radiate in Slovenian: Elektromagnetno
valovanje
radiate in Serbian: Електромагнетно
зрачење
radiate in SerboCroatian: Elektromagnetsko
zračenje
radiate in Sundanese: Gelombang
éléktromagnétik
radiate in Finnish: Sähkömagneettinen
säteily
radiate in Swedish: Elektromagnetisk
strålning
radiate in Tamil: மின்காந்த அலைகள்
radiate in Thai: คลื่นแม่เหล็กไฟฟ้า
radiate in Vietnamese: Bức xạ điện từ
radiate in Turkish: Elektromanyetik ışın
radiate in Ukrainian: Електромагнітна
хвиля
radiate in Contenese: 電磁波
radiate in Chinese: 电磁波
radiate in Slovak: Elektromagnetické
žiarenie
Synonyms, Antonyms and Related Words
be bright, beacon, beam, bedazzle, bestrew, blaze, blind, broadcast, burn, circulate, circumfuse, coruscate, daze, dazzle, deal out, diffract, diffuse, diffuse light, dispense, disperse, dispread, disseminate, distribute, diverge, emanate, emit, fan out, flame, flare, flash, fulgurate, give light, give
off, give out, glance,
glare, gleam, glint, glisten, glitter, glow, incandesce, irradiate, issue, jam, luster, newscast, overscatter, oversow, overspread, propagate, publish, radio, radiobroadcast, ray, retail, scatter, scintillate, send, send out, send out rays,
shed, shimmer, shine, shine brightly, shoot, shoot out rays, shortwave, sign off, sign on,
sow, sow broadcast, sparkle, splay, sportscast, spread, spread out, strew, transmit, twinkle, utter, wireless