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[[pt:Energia radiante]]
[[sk:Žiarivá energia]]
{{çeviri}}
[[Image:Trees and sunshine.JPG|thumb|right|333px|Light (a form of radiant energy) observed in a forest]]
'''Radiant energy''' is the [[energy]] of [[electromagnetic wave]]s.<ref>"''[http://www.its.bldrdoc.gov/fs-1037/dir-029/_4341.htm Radiant energy]''". [[Federal standard 1037C]]</ref> The quantity of radiant energy may be calculated by [[Integral|integrating]] [[radiant flux]] (or [[Power (physics)|power]]) with respect to [[time]] and, like all forms of energy, its [[SI]] unit is the [[joule]]. The term is used particularly when radiation is emitted by a source into the surrounding environment. Radiant energy may be visible or invisible to the human eye.<ref>George Frederick Barker, ''Physics: Advanced Course'', page 367</ref><ref>Hardis, Jonathan E., "[http://nvl.nist.gov/pub/nistpubs/sp958-lide/025-027.pdf Visibility of Radiant Energy]". [[Portable Document Format|PDF]].</ref>
==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]]). 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, the term "electro-radiant energy" has also been used.<ref>Examples: {{patent|US|1005338|"Transmitting apparatus"}}, {{patent|US|1018555|"Signaling by electroradiant energy"}}, and {{patent|US|1597901|"Radio apparatus"}}.</ref>
Historically, the propagation of electromagnetic radiation was presumed to rely on a medium filling all space, known as the [[aether theories|aether]].<ref>Thomas Preston, "''[http://books.google.com/books?vid=0Ip-GtK-wSYf2ItVGW&id=QgseAAAAMAAJ The Theory of Light]''". Macmillan, 1901. Page 542.</ref><ref>{{cite book |first=George Frederick |last=Barker |title=Physics: Advanced Course |url=http://books.google.com/books?vid=0hXR-OKgxbYIeyr8l5-&id=bKEAAAAAMAAJ |publisher=Henry Holt and Company |year=1893 |pages=365}}</ref><ref>Frederick Booth, ''Radiant Energy and the Ophthalmic Lens''.</ref> Electromagnetic waves were presumed to propagate through this medium by inducing transverse electric and magnetic stresses and strains, analogous to those induced by [[S-wave|shear wave]]s propagating through a physical medium.<ref>{{cite book| first=Louis| last=Bell| title=Electric Power Transmission; a Practical Treatise for Practical Men| publisher=Electrical World and Engineer| year=1901 |pages=10 |url=http://books.google.com/books?id=hSYKAAAAIAAJ&pg=RA3-PA110&lpg=RA3-PA110&dq=%22electric+power+transmission+a+practical+treatise+for+practical+men%22&source=web&ots=FTKTW8smJm&sig=8kcwxaAWKmm5-1ysBFR52oQiRik#PRA1-PA10,M1 |accessdate=2007-02-15}}</ref> In modern times, the propagation of electromagnetic waves has been shown not to require any physical medium, although some interpretations of [[general relativity]] can be viewed as implying that space acts as a kind of non-physical "medium" for light.<ref>[[Albert Einstein]] said that space is "endowed with physical quantities," but that "this ether may not be thought of as endowed with the quality characteristic of ponderable media [...] The idea of motion may not be applied to it." (from "[http://www.mountainman.com.au/aether_0.html Ether and the Theory of Relativity]", an address delivered May 5, 1920 at the University of Leyden).</ref><ref>[[Paul Dirac|P.A.M. Dirac]], "Is there an ether?" ''Nature'', '''168''', 906 (1951). Dirac wrote, "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."</ref>
==Analysis==
[[Image:TrigaReactorCore.jpeg|thumb|333px|[[Cherenkov radiation]] glowing in the core of a [[TRIGA|TRIGA reactor]].]]
Because electromagnetic (EM) radiation can be conceptualized as a stream of [[photon]]s, 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 [[frequency|frequencies]]. The bands of frequency present in a given EM signal may be sharply defined, as is seen in [[Emission spectrum|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 (physics)|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 [[Absorption (optics)|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 [[Reflection (physics)|reflected]] or [[scattering|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 (systems theory)|open system]].<ref>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.</ref><ref>Robert W. Christopherson, ''Elemental Geosystems'', Fourth Edition. Prentice Hall, 2003. Pages 608. ISBN 0131015532</ref><ref>James Grier Miller and Jessie L. Miller, ''[http://www.newciv.org/ISSS_Primer/asem22jm.html The Earth as a System]''.</ref> Such a system can be man-made, such as a [[solar energy]] collector, or natural, such as the [[Earth's atmosphere]]. In [[geophysics]], most atmospheric gases, including the [[greenhouse gas]]es, allow the Sun's short-wavelength radiant energy to pass through to the Earth's surface, heating the ground and oceans. The absorbed solar energy is partly re-emitted as longer wavelength radiation (chiefly infrared radiation), some of which is absorbed by the atmospheric greenhouse gases. Radiant energy is produced in the sun as a result of [[nuclear fusion]].<ref>''[http://assets.cambridge.org/97805217/91656/excerpt/9780521791656_excerpt.pdf Energy transformation]''. assets.cambridge.org. (excerpt)</ref>
==Applications==
Radiant energy, as well as [[convective|convective energy]] and [[conductive|conductive energy]], is used for [[radiant heating]].<ref>{{patent|US|1317883|"Method of generating radiant energy and projecting same through free air for producing heat"}}</ref> 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).
[[Image:PhotoelectricEffect(Tesla).png|thumb|333px|right|Photoelectric motor, US685957<br>
Radiant energy falling on a insulated conductor connected to a capacitor: the capacitor charges electrically.]]
Various other applications of radiant energy have been devised.<ref> [http://www.uspto.gov/go/classification/uspc250/defs250.htm Class 250, Radiant Energy], USPTO. March 2006.</ref> These include:
* Treatment and inspection
* Separating and sorting
* Medium of control
* Medium of communication
Many of these applications involve a source of radiant energy and a detector that responds to that radiation and provides a signal representing some characteristic of the radiation. Radiant energy detectors produce responses to incident radiant energy either as an increase or decrease in [[electric potential]] or [[electric current|current]] flow or some other perceivable change, such as exposure of [[photographic film]].
One of the earliest wireless [[telephone]]s 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.<ref name="Anderson">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.</ref> 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}}
==See also==
{{col-begin}}
{{col-2}}
*[[Luminous energy]]
*[[Power (physics)|Power]]
*[[Radiometry]]
*[[Federal Standard 1037C]]
*[[Transmission (telecommunications)|Transmission]]
*[[Open system (systems theory)|Open system]]
{{col-break}}
{{portal|Energy}}
*[[Photoelectric effect]]
*[[Photodetector]]
*[[Photocell]]
*[[Photoelectric cell]]
{{col-end}}
{{radio_spectrum}}
{{EMSpectrum}}
==Notes and references==
{{refbegin}}
<references/>
* {{cite book |last=Lang |first=Kenneth R. |year=1999 |url=http://books.google.com/books?id=HlGIXqzVEAgC |title=Astrophysical Formulae |location=Berlin |publisher=Springer}}
* {{cite web |url=http://www.schorsch.com/kbase/glossary/radiant_energy.html |title=Radiant energy |work=Lighting Design Knowledgebase |first=Georg |last=Mischler |year=2003 |accessdate=29 Oct. 2008}}
* {{cite book |first=Glenn R. |last=Elion |url=http://books.google.com/books?id=c1jqBKKKDkwC |title=Electro-Optics Handbook |publisher=CRC Press Technology & Industrial Arts |year=1979 |isbn=0824768795}}
{{refend}}
==Further reading==
{{refbegin}}
* Caverly, Donald Philip, ''Primer of Electronics and Radiant Energy''. New York, McGraw-Hill, 1952.
* {{cite journal |last=Whittaker |first=E. T. |url=http://links.jstor.org/sici?sici=0025-5572(192904)2%3A14%3A200%3C401%3AWIE%3E2.0.CO%3B2-K |title=What is energy? |journal=The Mathematical Gazette |year=1929 |volume=14 |issue=200 |month=Apr. |pages=401–406 |publisher=The Mathematical Association |doi=10.2307/3606954}}
{{refend}}
<!--Categories-->
[[Category:Electromagnetic radiation]]
[[Category:Radiometry]]
[[ar:طاقة إشعاعية]]
[[ca:Energia radiant]]
[[cs:Energie záření]]
[[de:Strahlungsenergie]]
[[el:Ενέργεια ακτινοβολίας]]
[[es:Energía electromagnética]]
[[eo:Radiada energio]]
[[fr:Énergie électromagnétique]]
[[it:Grandezze radiometriche]]
[[ja:放射エネルギー]]
[[pt:Energia radiante]]
[[sk:Žiarivá energia]]
[[tr:Işınım erkesi]]
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