Schematic of a Schlieren set up. Coherent light passes from right to left: HeNe laser source; microscopy lens to focus light onto a pinhole; metal disk with transparent pinhole; converging lens to make light parallel; water tank in which the sample (yellow) is placed and is impinged by ultrasound (blue arrow); lens to converge light to a spatial filter (e.g. a black ink spot on a transparent glass plate, which blocks undiffracted light); projection screen on which the sound beam (blue arrow, formed by diffracted light not blocked by the spatial filter) becomes visible and can be photographed.