Flow visualization with stereo shadowgraphs of stratified fluid.

Water stratified thermally or with dissolved material has a refractive index that varies with height. The stratification has no obvious visual effect until the fluid is disturbed, for example by a fish, but then it marks the creature's wake with a jump in refractive index that is visible in shadowgraphic projection. Two shadowgraphs in stereo display the wake in three dimensions. For a permanent record, or for analysing events too quick for the eye to follow, movies can be taken of the shadowgraphs.