$\frac{{i}_{s}}{{I}_{0}}=\frac{2{\text{\pi}}^{2}{[n(dn/dc)]}^{2}{k}_{B}Tc}{{r}^{2}{\lambda}^{4}{(\partial {\text{\pi}}_{osm}/\partial c)}_{0,T}}(1+{cos}^{2}{\varphi}_{x})$ |
(20) |

In Chapter 3 we developed expressions for the equilibrium osmotic pressure of a solution as a function of its concentration. Equation (3.34) may be written

${\text{\pi}}_{osm}=RT\left(\frac{c}{M}+B{c}^{2}\right)$ |
(21) |

Since Equation (21) applies at equilibrium, we may evaluate (*∂π*_{osm}*/∂c*)_{0,T} from Equation (21):

${\left(\frac{\partial {\text{\pi}}_{osm}}{\partial c}\right)}_{0,T}=RT\left(\frac{1}{M}+2Bc\right)$ |
(22) |

Combining Equations (20) and (22) yields

$\frac{{i}_{s}}{{I}_{0}}=\frac{2{\text{\pi}}^{2}{[n(dn/dc)]}^{2}c}{{N}_{A}{r}^{2}{\lambda}^{4}(1/M+2Bc)}(1+{cos}^{2}{\varphi}_{x})$ |
(23) |

Before looking at the experimental aspects of light scattering, it is convenient to define several more quantities. First, a quantity known as the Rayleigh ratio *R*_{θ} is defined as

${R}_{\theta}=\frac{{i}_{s}{r}^{2}}{{I}_{0}(1+{cos}^{2}\theta )}$ |
(24) |

where *θ* is the value of *ϕ*_{x} measured in the horizontal plane. According to Equation ...

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