CRITICAL BEHAVIOR OF THE REFRACTIVE-INDEX IN A BINARY FLUID

We have performed very accurate interferometer measurements of the refractive index n(precision ±6×10-7) in the critical mixture nitroethane-iso-octane. n depends on the density ρ and local field effects F. Even though this mixture is expected to show a very small anomaly in Δρ/ρ (exponent α) so that local field effects should have been observed, in fact we never saw a contribution due to F and the critical behavior of n is governed by the density anomaly alone. Using the matching of refractive index components and of thermal derivatives, the regular part of n can be evaluated and the value α = 0.10±0.02 is found, in agreement with the group renormalization value 0.110. The corresponding amplitude is compatible with the value obtained from ξ0 and (∂T c/∂p)V,xc measurements using universal amplitude relations. © 1979 American Institute of Physics.