View source for Radiant energy

{{Short description|Energy carried by electromagnetic or gravitational radiation}}
{{distinguish|Thermal radiation}}
[[File:Solar energy.jpg|thumb|upright=1.3|[[Visible light]] such as [[sunlight]] carries radiant energy, which is used in [[solar power]] generation.]]
In [[physics]], and in particular as measured by [[radiometry]], '''radiant energy''' is the [[energy]] of [[electromagnetic radiation|electromagnetic]]<ref>"''[https://web.archive.org/web/20190514214022/http://www.its.bldrdoc.gov/fs-1037/dir-029/_4341.htm Radiant energy]''". [[Federal standard 1037C]]</ref> and [[gravitational radiation]]. As energy, its SI unit is the [[joule]] (J). The quantity of radiant energy may be calculated by [[Integral|integrating]] [[radiant flux]] (or [[radiant flux|power]]) with respect to [[time]]. The symbol ''Q''<sub>e</sub> is often used throughout literature to denote radiant energy ("e" for "energetic", to avoid confusion with photometric quantities). In branches of physics other than radiometry, electromagnetic energy is referred to using ''E'' or ''W''. The term is used particularly when electromagnetic radiation is emitted by a source into the surrounding environment. This radiation may be visible or invisible to the human eye.<ref>George Frederick Barker, ''Physics: Advanced Course'', page 367</ref><ref>Hardis, Jonathan E., "[https://nvlpubs.nist.gov/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>

The term "radiant energy" also applies to [[gravitational radiation]].<ref>{{cite book|last1=Kennefick|first1=Daniel|title=Traveling at the Speed of Thought: Einstein and the Quest for Gravitational Waves|publisher=[[Princeton University Press]]|isbn=978-0-691-11727-0|url=https://books.google.com/books?id=UgggNsJhCp8C&q=gravitational+waves+radiant+energy&pg=PA97|access-date=9 March 2016|date=2007-04-15}}</ref><ref>{{cite journal|last1=Sciama|first1=Dennis|title=Cutting the Galaxy's losses|journal=New Scientist|date=17 February 1972|page=373|url=https://books.google.com/books?id=_BnfnEMifSoC&q=radiant+energy&pg=PA373|access-date=9 March 2016}}{{Dead link|date=March 2024 |bot=InternetArchiveBot |fix-attempted=yes }}</ref> For example, the [[First observation of gravitational waves|first gravitational waves ever observed]] were produced by a black hole collision that emitted about 5.3{{e|47}} joules of gravitational-wave energy.<ref>{{cite journal|last1=Abbott|first1=B.P.|title=Observation of Gravitational Waves from a Binary Black Hole Merger|journal=Physical Review Letters|volume=116|issue=6|pages=061102|date=11 February 2016|doi=10.1103/PhysRevLett.116.061102|pmid=26918975|bibcode=2016PhRvL.116f1102A |doi-access=free|arxiv=1602.03837}}</ref>

==Analysis==
[[File:TrigaReactorCore.jpeg|thumb|upright=1.2|[[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 [[photon energy]] – 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]]).{{Citation needed|date=October 2021}}

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 particle 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]]{{Citation needed|date=October 2021}}. 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 converted to [[heat]] (or converted to electricity in case of a [[photoelectric effect|photoelectric]] material). 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|0-471-27471-2}}.</ref><ref>Robert W. Christopherson, ''Elemental Geosystems'', Fourth Edition. Prentice Hall, 2003. Pages 608. {{ISBN|0-13-101553-2}}</ref><ref>James Grier Miller and Jessie L. Miller, ''[http://www.newciv.org/ISSS_Primer/asem22jm.html The Earth as a System] {{Webarchive|url=https://web.archive.org/web/20210422063016/http://www.newciv.org/ISSS_Primer/asem22jm.html |date=2021-04-22 }}''.</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 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 lamp]]s, 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. Because of this, the air temperature may be lower than in a conventionally heated building, even though the room appears just as comfortable.

Various other applications of radiant energy have been devised.<ref>[http://www.uspto.gov/go/classification/uspc250/defs250.htm Class 250, Radiant Energy] {{Webarchive|url=https://web.archive.org/web/20090703080850/http://www.uspto.gov/go/classification/uspc250/defs250.htm |date=2009-07-03 }}, USPTO. March 2006.</ref> These include treatment and inspection, separating and sorting, medium of control, and 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]].

==SI radiometry units==
{{SI radiometry units}}

==See also==
{{portal|Energy}}
{{div col|colwidth=22em}}
*[[Luminous energy]]
*[[Luminescence]] 
*[[Power (physics)|Power]]
*[[Radiometry]]
*[[Federal Standard 1037C]]
*[[Transmission (telecommunications)|Transmission]]
*[[Open system (systems theory)|Open system]]
*[[Photoelectric effect]]
*[[Photodetector]]
*[[Photocell]]
*[[Photoelectric cell]]
{{div col end}}

==Notes and references==
{{Reflist}}
{{refbegin}}
*{{cite book |last=Lang |first=Kenneth R. |date=1999 |url=https://books.google.com/books?id=HlGIXqzVEAgC |title=Astrophysical Formulae |location=Berlin |publisher=Springer | isbn=978-3-540-29692-8}}
*{{cite web |url=http://www.schorsch.com/kbase/glossary/radiant_energy.html |title=Radiant energy |work=Lighting Design Knowledgebase |first=Georg |last=Mischler |date=2003 |access-date=29 Oct 2008}}
*{{cite book |first=Glenn R. |last=Elion |url=https://books.google.com/books?id=c1jqBKKKDkwC |title=Electro-Optics Handbook |publisher=CRC Press Technology & Industrial Arts |date=1979 |isbn=0-8247-6879-5}}
{{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. |title=What is energy? |journal=The Mathematical Gazette |date=Apr 1929 |volume=14 |issue=200 |pages=401–406 |publisher=The Mathematical Association |doi=10.2307/3606954 |jstor=3606954|s2cid=187889019 }}
{{refend}}

{{radio spectrum}}
{{EMSpectrum}}
{{Footer energy}}

<!--Categories-->
[[Category:Electromagnetic radiation]]
[[Category:Radiometry]]
[[Category:Forms of energy]]