Thermal Radiation
Thermal radiation is defined as thermal (heat) energy transformations caused by the emission of electromagnetic waves from an emitting body. Thermal radiation summed over all wavelengths obeys the Stefan-Boltzmann law, which states that the radiation by a blackbody radiator per second per unit area is proportional to the fourth power of the absolute temperature. Mathematically, it is expressed as: P/A = T4 (in units of J/m 2 s), where P is the radiated power, A is the radiative area and is Stefan's constant, equal to 5.6703 x 10-8 watt/m 2 K4 . For radiating bodies other than ideal blackbody radiators, the expression is modified to: P/A = e T4 where e is the emissivity of the body; e = 1 for a blackbody. If the temperature of the radiating body is higher than that of its surroundings, the net radiation loss rate takes is given by: P = e A (T4 - T4 ) where T is the temperature of the radiating body and T is that of the surroundings. In accord with the laws of thermodynamics, the natural transfer of heat between two systems occurs from a higher to a lower temperature.
In the process, the internal energy of both systems is changed.
If the temperature of a tungsten filament is gradually increased, it begins to radiate photons and glow a dull red color at about 900K. As the temperature is further increased, the filament glows bright red, then orange, then yellow until it finally glows white, at which point the temperature is 2,300K. These types of phenomena were the object of what became one of the fundamental breakthroughs of quantum theory, in that classical physics was incapable of accounting for blackbody radiation. A perfect absorber, or blackbody, is a material which absorbs all frequencies of electromagnetic radiation and emits none. At the turn of the century, the experimentally observed variation of the radiated power density, or radiant excitance, of a blackbody with wavelength could not be explained by any model because classical theory assumed that energy was divided equally between all the vibrations emitting the radiation, with the result that the energy should increase as the wavelength became shorter, as short wavelengths have more vibrational modes. The problem was solved in 1901 German physicist Maxwell Planck proposed that energy consisted of discrete units, or quanta. From the assumption that the electromagnetic photons were thus quantized in energy (with the quantum of energy equal to the product of Plancküs constant and the frequency) Planck derived a radiation formula stating that the average energy per quantum is the energy of the quantum times the probability that it will be occupied in accordance with the Bose-Einstein distribution function.
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