Approximation of Radiation. Optical Rays

Radiation energy Solid angle x Area x Tune 

To find the effect of radiation on thermal problems, however, we need to interpret the optical rays diverging in all directions in terms of thermal concepts such as energy density and heat flux. First, introduce the definition of the energy density of radiation, 

introduce the emissive power E, or the radiation flux from an (actual) surface JA, as shown in Hg. 8.5. It follows that 

Equation (8.19), together with Eq. (8.16), show how thermal concepts such as u and Eb are related to the optical concept of I. 

Finally, inserting Eq. (8.7) into Eq. (8.16) and the result into Eq. (8.19) yields the Stefan-Boltzmann law, 

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