Radiant energy: Wikis


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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. 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.[2][3]


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.[4]

Historically, the propagation of electromagnetic radiation was presumed to rely on a medium filling all space, known as the aether.[5][6][7] Electromagnetic waves were presumed to propagate through this medium by inducing transverse electric and magnetic stresses and strains, analogous to those induced by shear waves propagating through a physical medium.[8] 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.[9][10]


Cherenkov radiation glowing in the core of a TRIGA reactor.

Because electromagnetic (EM) radiation can be conceptualized 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.[11][12][13] 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 gases, 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.[14]


Radiant energy, as well as convective energy and conductive energy, is used for radiant heating.[15] 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).

Photoelectric motor, US685957
Radiant energy falling on a insulated conductor connected to a capacitor: the capacitor charges electrically.

Various other applications of radiant energy have been devised.[16] 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 current flow or some other perceivable change, such as exposure of photographic film.

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.[17] 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
Quantity Symbol SI unit Abbr. Notes
Radiant energy Q joule J energy
Radiant flux Φ watt W radiant energy per unit time, also called radiant power
Radiant intensity I watt per steradian W·sr−1 power per unit solid angle
Radiance L watt per steradian per square metre W·sr−1·m−2 power per unit solid angle per unit projected source area.

called intensity in some other fields of study.

Irradiance E, I watt per square metre W·m−2 power incident on a surface.

sometimes confusingly called "intensity".

Radiant exitance /
Radiant emittance
M watt per square metre W·m−2 power emitted from a surface.
Radiosity J or Jλ watt per square metre W·m−2 emitted plus reflected power leaving a surface
Spectral radiance Lλ
watt per steradian per metre3

watt per steradian per square
metre per hertz



commonly measured in W·sr−1·m−2·nm−1

Spectral irradiance Eλ
watt per metre3
watt per square metre per hertz
commonly measured in W·m−2·nm−1

See also

Notes and references

  1. ^ "Radiant energy". Federal standard 1037C
  2. ^ George Frederick Barker, Physics: Advanced Course, page 367
  3. ^ Hardis, Jonathan E., "Visibility of Radiant Energy". PDF.
  4. ^ Examples: US patent 1005338 "Transmitting apparatus", US patent 1018555 "Signaling by electroradiant energy", and US patent 1597901 "Radio apparatus".
  5. ^ Thomas Preston, "The Theory of Light". Macmillan, 1901. Page 542.
  6. ^ Barker, George Frederick (1893). Physics: Advanced Course. Henry Holt and Company. pp. 365. http://books.google.com/books?vid=0hXR-OKgxbYIeyr8l5-&id=bKEAAAAAMAAJ. 
  7. ^ Frederick Booth, Radiant Energy and the Ophthalmic Lens.
  8. ^ Bell, Louis (1901). Electric Power Transmission; a Practical Treatise for Practical Men. Electrical World and Engineer. pp. 10. 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. Retrieved 2007-02-15. 
  9. ^ 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 "Ether and the Theory of Relativity", an address delivered May 5, 1920 at the University of Leyden).
  10. ^ 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."
  11. ^ 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.
  12. ^ Robert W. Christopherson, Elemental Geosystems, Fourth Edition. Prentice Hall, 2003. Pages 608. ISBN 0131015532
  13. ^ James Grier Miller and Jessie L. Miller, The Earth as a System.
  14. ^ Energy transformation. assets.cambridge.org. (excerpt)
  15. ^ US patent 1317883 "Method of generating radiant energy and projecting same through free air for producing heat"
  16. ^ Class 250, Radiant Energy, USPTO. March 2006.
  17. ^ 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.

Further reading



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