Superior mirage effect in supercritical CO2: Experiment and model

At temperatures near Tc, supercritical CO2 is highly compressible and it follows that a small temperature gradient will create a large density gradient. We established stable density gradients by maintaining the bottom of a sc-CO2-cell cooler than its other sides. That created a large refractive index gradient that bent a laser beam through an angle that is roughly proportional to the density gradient. Such light bending causes, for example, the mirage of water on a hot road. A He-Ne laser beam traversed a 7.62 cm long cell and was deflected downward as much as 5.9°. The magnitude of the beam deflections agreed within 3% with a model that used only fundamental geometrical optics and known PVT data, but that employed no adjustable parameters.