Observation of mean path length invariance in light-scattering media

Scattered light, it is all the same Materials can vary from transparent to opaque depending on the density of scatters within the medium. As light propagates through a material, intuition might suggest that the more scatters there are, the shorter the path along which the light can propagate. Savo et al. confirm a recent theoretical proposal that predicts that this is not the case. They shone light through a series of samples of varying scatterer density and found that the average path length that the light traveled was independent of the sample microstructure. This finding should also be applicable to acoustics and matter waves. Science, this issue p. 765 The average path length of light through a disordered medium is independent of the medium’s microstructure. The microstructure of a medium strongly influences how light propagates through it. The amount of disorder it contains determines whether the medium is transparent or opaque. Theory predicts that exciting such a medium homogeneously and isotropically makes some of its optical properties depend only on the medium’s outer geometry. Here, we report an optical experiment demonstrating that the mean path length of light is invariant with respect to the microstructure of the medium it scatters through. Using colloidal solutions with varying concentration and particle size, the invariance of the mean path length is observed over nearly two orders of magnitude in scattering strength. Our results can be extended to a wide range of systems—however ordered, correlated, or disordered—and apply to all wave-scattering problems.