Black Body Radiation and the Stefan-Boltzmann Law

Black-body Radiation and the Stefan-Boltzmann Law.—Light is simply electromagnetic radiation, a wave motion in space, in which the 

Ordinary bodies at any temperature above the absolute zero automatically emit radiation, and are capable of absorbing radiation falling on them. Thus an enclosure containing bodies at a temperature above the absolute zero cannot be in equilibrium unless it contains radiation as well. In fact, in equilibrium, there must be just enough radiation so that each square centimeter of surface of each body emits just as much radiation as it absorbs. It seems clear that there must be a definite sort of radiation in equilibrium with bodies at a definite temperature. For we 

Equation (1.2) expresses KirchhofTs law the ratio of the emissive power to the absorptivity of all bodies at the same wave length and temperature is the same. Put more simply, good radiators are good absorbers, poor radiators are poor absorbers. There are many familiar examples of this law. One, which is of particular importance in spectroscopy, is the following if an atom or other system emits a particularly large amount of 

Now that we understand the emissive power and absorptivity of bodies, we should consider Jx, the universal function of wave length and temperature describing black-body radiation. It is a little more convenient not to use this quantity, but a closely related one, u . This represents, not the energy falling on 1 sq. cm. per second, but the energy contained in a cubic centimeter of volume, or what is called the energy 

From Eq. (1.3) we can easily piove a law called the Stefan-Boltzmann law relating the density of radiation to the temperature. 

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