We determine the explicit timehyphen;dependence of a state function which initially corresponds to a molecule interacting with a single planehyphen;polarized photon. It turns out that two conditions must be met if the polarization of the photon is to remain linear but may change its orientation. One is automatically satisfied for freely oriented molecules in the absence of damping and external fields; the other yields an expression for the optical rotatory power. This expression can be applied to molecules of any size and is therefore a generalization of the Rosenfeld equation which is valid only in the longhyphen;wavelength limit. In the other extreme, ohgr;rarr;infin;, the optical activity also approaches zero, but in a manner quite different from the one that is sometimes discussed.
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