Some of the physical operations that can be performed on a beam of light are reversible; others are irreversible. In many situations the distinction is obvious. In other situations, especially where no absorption occurs, it may not be obvious whether a postulated operation is reversible. An experimenter might try to produce a result that is basically impossible of achievement.
Fortunately, simple thermodynamic considerations provide a complete interpretation. Thermodynamic considerations are applicable because all sources of visible light are thermal in nature. Planck’s formula for blackbody radiation leads directly to once well-known expressions for the temperature and entropy flux of a beam of light of given wavelength, direction, polarization, and spectral radiance. By analyzing the temperature and entropy changes involved in any given optical operation, the reversibility or irreversibility is easily established. The method is applied to various principal types of optical operations, including those entailing reflecting, condensing, dispersing, diffusing, beam-splitting, polarizing, and depolarizing.
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