Rosseland approximation

Summary

The Rosseland approximation, or diffusion approximation, treats radiative heat transfer in an optically thick medium like heat conduction. When radiation is absorbed and re-emitted many times over short distances, the radiative heat flux becomes proportional to the temperature gradient, as in Fourier's law. The radiative conductivity is $16 \sigma T^3 / (3 \beta_R)$, where $\beta_R$ is the Rosseland mean extinction coefficient. It is used for stellar interiors, where it defines the Rosseland mean opacity, and for insulating materials and glass melts.