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Radiation properties
Conduction and convection are heat transfer processes that require the presence of a medium.〔Hsu, Shao Ti. ''Engineering Heat Transfer.'' Blacksburg, Virginia:D. Van Nostrand Company, Inc.,1962.〕
Radiation heat transfer is characteristically different from the other two in that it does not require a medium and, in fact it reaches maximum efficiency in a vacuum. Electromagnetic radiation has some proper characteristics depending on the frequency and wavelengths of the radiation. The phenomenon of radiation is not yet fully understood. Two theories have been used to explain radiation; however neither of them is perfectly satisfactory.
First, the earlier theory which originated from the concept of a hypothetical medium referred as ether. Ether supposedly fills all evacuated or non evacuated spaces. The transmission of light or of radiant heat are allowed by the propagation of electromagnetic waves in the ether.〔 Electromagnetic waves have similar characteristics to television and radio broadcasting waves they only differ in wavelength.〔Becker, Martin. ''Heat Transfer a Modern Approach'' New York: Plenum Publishing Corporation, 1986.〕 All electromagnetic waves travel at the same speed; therefore, shorter wavelengths are associated with high frequencies. Since every body or fluid is submerged in the ether, due to the vibration of the molecules, any body or fluid can potentially initiate an electromagnetic wave. All bodies generate and receive electromagnetic waves at the expense of its stored energy 〔 The second theory of radiation is best known as the quantum theory and was first offered by Max Planck in 1900.〔 According to this theory, energy emitted by a radiator is not continuous but is in the form of quanta. Planck claimed that quantities had different sizes and frequencies of vibration similarly to the wave theory.〔Yunus, Cengel. ''Heat and Mass Transfer.''New York: Mc Graw Hill, 2007.〕 The energy E is found by the expression E = hν, where h is the Planck's constant and ν is the frequency. Higher frequencies are originated by high temperatures and create an increase of energy in the quantum.
The propagation of electromagnetic waves of all wavelengths is often referred as "radiation" thermal radiation is constrained to the visible and infrared regions. However, the term radiation refers to thermal radiation only. For engineering purposes, it may be stated that thermal radiation is a form of electromagnetic radiation which varies on the nature of a surface and its temperature.〔 Radiation waves may travel in unusual patterns compare to conduction heat flow. Radiation allows waves to travel from a heated body through a cold nonabsorbing or partially absorbing medium and reach a warmer body again.〔 This is the case of the radiation waves that travel from the sun to the earth.
==Properties==
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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