Black Body Radiation

In general the radiation given off by a body depends on many things. It is possible to construct a model for a perfect emitter of radiation. The perfect emitter will also be a perfect absorber, absorbing all the radiation incident on it and reflecting none. For this reason it is called a black body, and the radiation is called black body radiation.

Black body radiation does not depend on the nature of the emitting surface, only on it's temperature. At any given temperature there will be a range of different wavelengths and frequencies emitted. The radiation corresponding to certain wavelengths will be more intense than others. The variation of intensity with wavelength is shown blow for different temperatures.

The visible spectrum drawn on the diagram illustrates that hotter bodies radiate more in the blue part of the spectrum and cooler bodies radiate more in the red part, and perhaps surprisingly, a hotter body will radiate more at the same temperature than a cooler body, since the intensity increases everywhere with increasing temperature, and also, the area under each temperature curve gives the total power emitted.

Although stars are not perfect emitters, the black body curve does give quite a good approximation to the radiation emitted by a star.

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