The scattering of light originates from the fluctuation in the permittivity that gives rise to a turbidity of the fluid. The turbidity of an otherwise homogeneous fluid is, according to Einstein's theory, proportional to the mean-square fluctuation in the relative permittivity. A general expression for the mean-square fluctuation or variance of a fluctuating property is derived. It shows that the turbidity depends on the second derivatives of an appropriately chosen state function - the choice is dependent on which properties that fluctuate. The paper provides a general derivation and outlines the criteria for the choice of state function. Data for light-scattering are modelled using the Clausius-Mossotti equation and an excess Gibbs energy function. The excess Gibbs energy applied consists of three terms: Flory-Huggins (accounting for the size of the components), Debye-Huckel (accounting for the charges of electrolytes) and Kirkwood (accounting for the dipolar nature of proteins). The present investigation shows that the model parameters determined by light-scattering experiments have a well-defined physical significance and light-scattering data can thus be used in parallel with other data to provide information from which model parameters can be estimated. (C) 2011 Elsevier B.V. All rights reserved.