To measure the velocity of the smallest particles suspended in the flowing fluid, an interference pattern is used, as shown in the figure.
A parallel beam of light from a laser with $\lambda = 630$ nm is incident on two identical prisms stacked on their bases (bi-prism). The refractive angle of each of the prisms is $\alpha = 5,7$ degrees and the refractive index is $n = 1.5$ After passing through the bi-prism, the light splits into two beams that pass through the cuvette with liquid. The particles, moving together with the liquid with some velocity v, scatter the light.