Two-Scale Factor Universality in O₂ and H₂

Video images are increasingly used in science, supplanting current methods such as light scattering by a statistical evaluation of the images. In this paper, it is shown that light turbidity data, a quantity due to density-induced refractive index fluctuations, can be directly obtained from image analysis. We fitted the turbidity data to a theoretical expression that allowed us to estimate the critical amplitudes of isothermal compressibility and correlation length. Images of an oxygen-filled cell and a hydrogen-filled cell were taken near their respective critical temperatures of 155 and 33 K. Images for Hydrogen were analyzed and values of isothermal compressibility and fluctuation correlation lengths obtained from turbidity fitting were compared against literature values. In order to go very close to the critical point, the fluids are placed under weightlessness using a magnetic levitation device. Our data analysis shows a large sensitivity of the fitting parameters to the refractive index value and to even minute density deviations from criticality.