Double-scattering-induced deviations from Ornstein-Zernike behavior near the critical point

We consider double light scattering by a simple fluid near its critical point. It is shown that Ornstein-Zernike-Debye plots of inverse total (single plus double) scattering intensity are not linear, but show downward curvature at small scattering angles near the critical point. This effect has been observed experimentally and is usually attributed to a nonzero value for the critical exponent $\eta$. We compare our conclusions with those of Splittorff and Miller, who have found that such downward curvature can also be caused by density gradients near the critical point.