radiant energy

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Light (a form of radiant energy) observed in a forest
Radiant energy is the energy of electromagnetic waves.[1] The quantity of radiant energy may be calculated by integrating radiant flux (or power) with respect to time and, like all forms of energy, its SI unit is the joule. Radiant energy is generally thought of as radiation emitted by a source into the surrounding environment. It propagates in the form of electromagnetic waves, or traveling subatomic, atomic or molecular particles. Specific forms of radiant energy include electron space discharge, visible light, vacuum energy, and other wave types.[2] Radiant energy is exhibited in the spontaneous nuclear disintegration with emission of particulate or electromagnetic radiations.

Terminology use and history

The term "radiant energy" is most commonly used in the fields of radiometry, solar energy, heating and lighting, but is also sometimes used in other fields (such as telecommunications). Radiant energy may or may not affect the eye and produce vision.[3] In modern applications involving transmission of power from one location to another, "radiant energy" is sometimes used to refer to the electromagnetic waves themselves, rather than their energy (a property of the waves). In the past, radiant energy has been called "electro-radiant energy".[4] The term and concept of "radiant energy" has historically also been applied to electrostatics.[5][6]

Historically, to account for the propagation of radiant energy — that is, light, actinic radiation, heat, electricity, cathode rays, X-rays, alpha rays, beta rays, and gamma rays — it was postulated the existence of a medium filling all space.[7][8][9] It was thought that both electrostatic and electromagnetic strains a medium, such as a aether, and the stresses in electro-magnetic energy are at right angles both to the electrostatic stresses and to the direction of their motion or flow.[10] When the material medium transmitting the radiation is at rest, the ray, or path of the radiant energy, is the same relative to the matter as to the aether.[11] In modern times, accounting for the propagation of radiant energy through the existence of a medium filling all space is possible [12] but not necessary as the electrodynamic effects and electromagnetic waves generally, it is thought, do not require a physical transmission medium unlike mechanical waves, and so can travel through the "vacuum" of free space. Regions of the insulative vacuum can become conductive for electrical conduction through the presence of free electrons, holes, or ions.


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Cherenkov radiation, a type of radiant energy, glowing in the core of a TRIGA reactor.
Because electromagnetic (EM) radiation can be conceptualized, in modern times, as a stream of photons, radiant energy can be viewed as the energy carried by these photons. Alternatively, EM radiation can be viewed as an electromagnetic wave, which carries energy in its oscillating electric and magnetic fields. These two views are completely equivalent and are reconciled to one another in quantum field theory (see wave-particle duality).

EM radiation can have various frequencies. The bands of frequency present in a given EM signal may be sharply defined, as is seen in atomic spectra, or may be broad, as in blackbody radiation. In the photon picture, the energy carried by each photon is proportional to its frequency. In the wave picture, the energy of a monochromatic wave is proportional to its intensity. This implies that if two EM waves have the same intensity, but different frequencies, the one with the higher frequency "contains" fewer photons, since each photon is more energetic.

When EM waves are absorbed by an object, the energy of the waves is typically converted to heat. This is a very familiar effect, since sunlight warms surfaces that it irradiates. Often this phenomenon is associated particularly with infrared radiation, but any kind of electromagnetic radiation will warm an object that absorbs it. EM waves can also be reflected or scattered, in which case their energy is redirected or redistributed as well.

Open systems

Radiant energy is one of the mechanisms by which energy can enter or leave an open system.[13][14][15] Such a system can be man-made, such as a solar energy collector, or natural, such as the Earth's atmosphere. In geophysics, transparent greenhouse gases trap the sun's radiant energy (at certain wavelengths), allowing it to penetrate deep into the atmosphere or all the way to the Earth's surface, where they are re-emitted as longer wavelength radiation (chiefly infrared radiation). Radiant energy is produced in the sun as a result of nuclear fusion.[16]


Radiant energy, as well as convective energy and conductive energy, is used for radiant heating.[17] It can be generated electrically by infrared lamps, or can be absorbed from sunlight and used to heat water. The heat energy is emitted from a warm element (floor, wall, overhead panel) and warms people and other objects in rooms rather than directly heating the air. The internal air temperature for radiant heated buildings may be lower than for a conventionally heated building to achieve the same level of body comfort (the perceived temperature is actually the same).

Various other methods and apparatus involving radiant energy have been devised.[18] All methods and apparatus using, generating, controlling or detecting radiant energy involve combinations of circuits which are closed or closable conducting paths through which, or along which, electric current can travel. In general, devices involving radiant energy is provided on the basis of either a specific use of the radiant energy or a specific type of radiant energy. Devices explicitly providing for subject matter involving radiant energy are:
  • Treatment and inspection
  • Separating and assorting
  • Medium of control
  • Medium of communication
Inspection is used to imply a source of radiant energy, and/or means to irradiate an object by said source and a detector responsive to radiation from the object to provide a signal representing some characteristic of the object. Various devices use material which when subjected to radiation for treatment or whose response to or effect on the radiation is used to indicate something about the material.

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Photoelectric motor, US685957
Radiant energy falling on a insulated conductor connected to a capacitor: the capacitor charges electrically.
Radiant energy detectors produce responses to incident radiant energy either as an increase or decrease in electric potential or current flow (Electric) or some other perceivable change (Nonelectric). The nonelectric change may be immediately perceived or may require development to be perceived, e.g., photographic changes. Since radiant energy is really just electromagnetic radiation under another name, it is the basis of a wide range of communication technologies using radiofrequency and microwave radiation. Radiant energy is observed by a detector, which is any material or device whose response to radiant energy is used to indicate the presence or amount of incident radiation. This is also called "Signalling Means". Some devices are detectors used to sense light incident thereon and generate a signal representative of some aspect of the light such as intensity, phase, coherence, mode distribution, and interference pattern characteristics.

One of the earliest wireless telephones to be based on radiant energy was invented by Nikola Tesla. The device used transmitters and receivers whose resonances were tuned to the same frequency, allowing communication between them. In 1916, he recounted an experiment he had done in 1896.[19] He recalled that "Whenever I received the effects of a transmitter, one of the simplest ways [to detect the wireless transmissions] was to apply a magnetic field to currents generated in a conductor, and when I did so, the low frequency gave audible notes."

SI radiometry units

SI radiometry units

[[ edit]]

Quantity Symbol SI unit Abbr. Notes
Radiant energyQjouleJenergy
Radiant fluxFwattWradiant energy per unit time, also called radiant power
Radiant intensityIwatt per steradianWsr−1power per unit solid angle
RadianceLwatt per steradian per square metreWsr−1m−2power per unit solid angle per unit projected source area.
Sometimes confusingly called "intensity".
IrradianceEwatt per square metreWm−2power incident on a surface.
Sometimes confusingly called "intensity".
Radiant exitance / Radiant emittanceMwatt per square metreWm−2power emitted from a surface.
RadiosityJ or Jλwatt per square metreWm−2emitted plus reflected power leaving a surface
Spectral radianceLλ
watt per steradian per metre3 or
watt per steradian per square metre per hertz
commonly measured in Wsr−1m−2nm−1
Spectral irradianceEλ
watt per metre3 or
watt per square metre per hertz
commonly measured in Wm−2nm−1

See also

Main concepts: Luminous energy, Power, Radiometry, Federal Standard 1037C, Transmission, Electrostatics, Ionizing radiation
Science:Photoelectric effect, Zero-point energy, Open system, Cosmic microwave background radiation, Schumann resonance
Photonic devices:Photodetector, Photocell, Photoelectric cell

Radio spectrum
3 Hz30 Hz300 Hz3 kHz30 kHz300 kHz3 MHz30 MHz300 MHz3 GHz30 GHz
30 Hz300 Hz3 kHz30 kHz300 kHz3 MHz30 MHz300 MHz3 GHz30 GHz300 GHz

References and further reading


1. ^ "Radiant energy". Federal standard 1037C
2. ^ This would include all electrical phenomena of transverse waves (those with vibrations perpendicular to the direction of the propagation) and longitudinal waves (those with vibrations parallel to the direction of the propagation).
3. ^ George Frederick Barker, Physics: Advanced Course. Page 367
4. ^ Examples of the term 'electroradiant energy' include U.S. Patent 1005338, 1018555, and 1597901.
5. ^ William H. Preece, "On the transmisson of electric signals through space", electrostatic induction, The Electrical engineer. (1884). London: Biggs & Co. Page 200
6. ^ Thomas Preston, "The Theory of Light". Macmillan, 1901. 586 pages. Page 544.
7. ^ Thomas Preston, "The Theory of Light". Page 542.
8. ^ George Frederick Barker, Physics: Advanced Course. Page 365
9. ^ Frederick Booth, Radiant Energy and the Ophthalmic Lens.
10. ^ Bell, Louis (1901). Electric Power Transmission; a Practical Treatise for Practical Men. Electrical World and Engineer, p. 10. Retrieved on 2007-02-15. 
11. ^ Sir Joseph Larmor, Aether and Matter. Page 30.
12. ^ Albert Einstein stressed that space is "endowed with physical quantities" He held that general relativity attributed tangible physical properties to space including some kind of medium for light, although not a material one. He stated in the 1920 paper, "Ether and the Theory of Relativity", "[the aether is] allowed to assume a space-filling medium if one can refer to electromagnetic fields (and thus also for sure matter) as the condition thereof". Additionally, Paul Dirac, in a 1951 article in Nature, titled "Is there an ether?" that "we are rather forced to have an ether"'. Dirac wrote about his theory: "We have now the velocity at all points of space-time, playing a fundamental part in electrodynamics. It is natural to regard it as the velocity of some real physical thing. Thus with the new theory of electrodynamics we are rather forced to have an ether."
13. ^ Moran, M.J. and Shapiro, H.N., ''Fundamentals of Engineering Thermodynamics, Chapter 4. "Mass Conservation for an Open System", 5th Edition, John Wiley and Sons. ISBN 0471274712.
14. ^ Robert W. Christopherson,
Elemental Geosystems, Fourth Edition. Prentice Hall, 2003. Pages 608. ISBN 0131015532
15. ^ James Grier Miller and Jessie L. Miller,
The Earth as a System.
16. ^
Energy transformation''. assets.cambridge.org. (excerpt)
17. ^ U.S. Patent 1317883 - Method of generating radiant energy and projecting same through free air for producing heat
18. ^ Class 250, Radiant Energy, USPTO. March 2006.
19. ^ Anderson, Leland I. (editor), Nikola Tesla On His Work With Alternating Currents and Their Application to Wireless Telegraphy, Telephony and Transmission of Power, 2002, ISBN 1-893817-01-6.

General Information


energy (from the Greek ενεργός, energos, "active, working")[1] is a scalar physical quantity that is a property of objects and systems of objects which is conserved by nature.
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Electromagnetic (EM) radiation is a self-propagating wave in space with electric and magnetic components. These components oscillate at right angles to each other and to the direction of propagation, and are in phase with each other.
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INTErnational Gamma-Ray Astrophysics Laboratory (INTEGRAL) is detecting some of the most energetic radiation that comes from space. It is the most sensitive gamma ray observatory ever launched.
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In radiometry, radiant flux or radiant power is the measure of the total power of electromagnetic radiation (including visible light). The power may be the total emitted from a source, or the total landing on a particular surface.
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In physics, power (symbol: P) is the rate at which work is performed or energy is transmitted, or the amount of energy required or expended for a given unit of time.
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One view is that time is part of the fundamental structure of the universe, a dimension in which events occur in sequence, and time itself is something that can be measured.
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Si, si, or SI may refer to (all SI unless otherwise stated):

In language:
  • One of two Italian words:
  • sì (accented) for "yes"
  • si

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The joule (IPA: [dʒuːl] or [dʒaʊl]) (symbol: J) is the SI unit of energy.
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Electric glow discharge is a type of plasma formed by passing a current at 100 V to several kV through a gas, usually argon or another noble gas. It is found in products such as fluorescent lights and plasma-screen televisions, and is used in plasma physics and analytical chemistry.
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visible spectrum (or sometimes optical spectrum) is the portion of the electromagnetic spectrum that is visible to (can be detected by) the human eye. Electromagnetic radiation in this range of wavelengths is called visible light or simply light.
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Vacuum energy is an underlying background energy that exists in space even when devoid of matter (known as free space). The vacuum energy results in the existence of most (if not all) of the fundamental forces - and thus in all effects involving these forces, too.
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Spontaneous emission is the process by which a light source such as an atom, molecule, nanocrystal or nucleus in an excited state undergoes a transition to the ground state and emits a photon.
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In nuclear science, the decay chain refers to the radioactive decay of different discrete radioactive decay products as a chained series of transformations. Most radioactive elements do not decay directly to a stable state, but rather undergo a series of decays until eventually a
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In physics, emission is the process by which the energy of a photon is released by another entity, for example, by an atom whose electrons make a transition between two electronic energy levels. The photon is created in the process.
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Particle radiation is the radiation of energy by means of fast-moving subatomic particles. Particle radiation is referred to as a particle beam if the particles are all moving in the same direction, similar to a light beam.
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Electromagnetic (EM) radiation is a self-propagating wave in space with electric and magnetic components. These components oscillate at right angles to each other and to the direction of propagation, and are in phase with each other.
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In optics, radiometry is the field that studies the measurement of electromagnetic radiation, including visible light. Note that light is also measured using the techniques of photometry, which deal with brightness as perceived by the human eye, rather than absolute power.
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Solar energy is energy from the sun. It supports life on Earth and drives the Earth's weather. Solar energy predominantly arrives in the form of infrared, visible and ultraviolet light, and is either returned back to space or is absorbed.
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Heating may refer to:
  • HVAC: Heating, ventilation and air-conditioning
Heating devices, or systems:
  • Block heater, or headbolt heater, an electric heater that heats the engine of a car to ease starting in cold weather
  • Boiler

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Lighting includes both artificial light sources such as lamps and natural illumination of interiors from daylight.
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Telecommunication is the transmission of signals over a distance for the purpose of communication. In modern times, this process typically involves the sending of electromagnetic waves by electronic transmitters, but in earlier times telecommunication may have involved the use of
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Electrostatics (also known as static electricity) is the branch of physics that deals with the phenomena arising from what seem to be stationary electric charges. This includes phenomena as simple as the attraction of plastic wrap to your hand after you remove it from a
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Light is electromagnetic radiation of a wavelength that is visible to the eye (visible light). In a scientific context, the word "light" is sometimes used to refer to the entire electromagnetic spectrum.
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Actinism is a property of radiation (particularly solar radiation) that leads to the production of photochemical effects. Actinism is derived from Greek with the meaning a radiant force.
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If you are prevented from editing this page, and you wish to make a change, please discuss changes on the talk page, request unprotection, log in, or .
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Electricity (from New Latin ēlectricus, "amberlike") is a general term for a variety of phenomena resulting from the presence and flow of electric charge. This includes many well-known physical phenomena such as lightning, electromagnetic fields and electric currents,
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Cathode rays are streams of electrons observed in vacuum tubes, i.e. evacuated glass tubes that are equipped with at least two electrodes, a cathode (negative electrode) and an anode (positive electrode) in a configuration known as a diode.
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X-rays (or Röntgen rays) are a form of electromagnetic radiation with a wavelength in the range of 10 to 0.01 nanometers, corresponding to frequencies in the range 30 PHz to 30 EHz. X-rays are primarily used for diagnostic radiography and crystallography.
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Alpha particles (named after and denoted by the first letter in the Greek alphabet, α) consist of two protons and two neutrons bound together into a particle identical to a helium nucleus; hence, it can be written as He2+ or 42He.
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Beta particles are high-energy, high-speed electrons or positrons emitted by certain types of radioactive nuclei such as potassium-40. The beta particles emitted are a form of ionizing radiation also known as beta rays. The production of beta particles is termed beta decay.
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