what is the difference between absortive power and the absorption coefficient of a surface for a radiation?

The emission spectrum of such a black body was first fully described by Max Planck. The energy emitted per unit time and surface [W m^-2] at some particular wavelength l was found to be given by:

(2.1)

Here the constant h is known as Plancks constant with h = 6,626 10 -34 [J s], furthermore k is Boltzmanns constant in [J K-1] and c is the speed of light with c = 3,0 108 [m s-1]. Finally the energy emitted also depends on the absolute temperature T of the emitting body.
The total radiation emitted per unit surface is found through integration of the intensity over all wavelengths, leading to the Stefan-Boltzmann law:

(2.2)

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