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By the Mid-Nineteenth century the wave theory of light was well established. One could thus consider relationships existing in a specified spectral region. In fact, the theorem of Gustav Robert Kirchhoff (1824 to 1887) considered the fraction of energy incident in a narrow spectral band on a cavity maintained at a given temperature. He showed that the ratio of the emission to the energy absorbed in that spectral band must be a constant, and that this constant represents an intensity distribution which is a universal function. Such a function is dependent only on the temperature and wavelength, and not on the size, shape, or material of the cavity.
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