  Our everyday experience teaches us that the structure of a medium strongly
influences how light propagates through it. A disordered medium, e.g., appears
transparent or opaque, depending on whether its structure features a mean free
path that is larger or smaller than the medium thickness. While the
microstructure of the medium uniquely determines the shape of all penetrating
light paths, recent theoretical insights indicate that the mean length of these
paths is entirely independent of any structural medium property and thus also
invariant with respect to a change in the mean free path. Here, we report an
experiment that demonstrates this surprising property explicitly. Using
colloidal solutions with varying concentration and particle size, we establish
an invariance of the mean path length spanning nearly two orders of magnitude
in scattering strength, from almost transparent to very opaque media. This very
general, fundamental and counterintuitive result can be extended to a wide
range of systems, however ordered, correlated or disordered, and has important
consequences for many fields, including light trapping and harvesting for solar
cells and more generally in photonic structure design.
