ENTROPY PRODUCTION IN MULTIPLE-SCATTERING OF LIGHT BY A SPATIALLY RANDOM MEDIUM

This study reports on the problem of entropy production due to multiple scattering of light by a spatially random medium composed of uncorrelated and noninteracting spherical dielectric particles. The degree of polarization P of light, in the form of plane waves, is of the nature of an order parameter for the ensemble of realizations of the fluctuating optical field. The radiation entropy takes a form analogous to the entropy of one-dimensional Ising (two-level) spin systems in contact with a heat bath. On the basis of this analysis, the degree of polarization has a different thermodynamic significance. It is argued that within this representation, one may define an effective polarization temperature; we then show how depends on the degree of polarization. Light transmitted through a multiple scattering medium is depolarized by decorrelation of the phases of the electric field components and its polarization entropy increases. The effects of size of the spherical particles and of the optical depth on entropy production are studied numerically, using the Mie theory, via the Monte Carlo method. An attempt is made to interpret these results in terms of the minimization procedure (minimum entropy production) that plays a fundamental role in classical irreversible thermodynamics. One of the most remarkable aspects of this problem, where no energy exchange between radiation and scatterer takes place, is that the stationary state corresponds both to the state of minimum production of radiation entropy and to the state of maximum entropy. Thermodynamically, multiple scattering can be viewed as an order-disorder transition using the spin model. It is also emphasized that the system will tend to evolve towards a higher polarization temperature state. We briefly comment on the use of our treatment in interpreting the irreversibility in a scattering process. © 1994 The American Physical Society.