Thermal Radiation from Partially Transparent Reflecting Bodies

The emissivity, transmissivity, and reflectivity are considered in detail for heated, reflecting, partially transparent bodies. Kirchhoff’s law, which in its usual form applies only to opaque bodies, is generalized to apply to matter of all kinds. An exact solution is found for the relationships between the “apparent reflectivity,” R*, and the true reflectivity and true transmissivity. Similarly, the “apparent transmissivity,” T*, is expressed as a function of the true transmissivity and true reflectivity. Further development of these considerations makes it possible to construct a ternary chart in which the observables E, T*, and R* are plotted at the apices of a triangle, the true transmissivities and reflectivities then appear as a system of intersecting lines on the chart.