Gravity effects on light scattering from a simple fluid near its critical point

The effect of the earth's gravity on light scattering from a simple fluid near its critical point is investigated. It is found that, in general, gravity influences the angular distribution of the scattered irradiance in three ways. First, the forward scattering at the critical point does not diverge. Second, the irradiance in any but the forward direction is not greatest at the critical temperature, but slightly above it. Third, Ornstein-Zernike-Debye plots of the inverse irradiance versus the square of the wave number are concave downward at small scattering angles. This last feature is usually attributed to a nonvanishing of the critical exponent $\eta$ and thus indicates that the experimental evidence for this conjecture is misleading. Computations are peformed using an approximate equation of state to exhibit these deviations from the traditional Ornstein-Zernike theory.