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Radiant energy is the energy of electromagnetic waves. 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, and other wave types. 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. 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". The term and concept of "radiant energy" has historically also been applied to electrostatics.
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. 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. 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. In modern times, accounting for the propagation of radiant energy through the existence of a medium filling all space is possible  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.
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.
Radiant energy is one of the mechanisms by which energy can enter or leave an open system. 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.
Radiant energy, as well as convective energy and conductive energy, is used for radiant heating. 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. 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:
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.
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. 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
References and further reading
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Radiant_energy". A list of authors is available in Wikipedia.|