Two monochromatic plane waves that passed through the same optical path and stemming from the same laser interfere at the level of the same laminar flow of a liquid. Wave vectors and of both waves do angle. The liquid flow is perpendicular to sum vector directed in function of the axis. Figure 52 illustrates the principle of the velocimeter.
A diffusing particle of about of diameter, dragged by a liquid crosses the interference area and diffuses light only when it is at the level of a bright fringe.A photodetector detects the light diffused by the particle and releases an electrical signal which frequency depends on the particle speed .
a) Find the intensity distribution in plane , describe the fringes aspect and calculate the interfringe.
b) Find the expression of fluid flow speed assuming that the particle perfectly follows that flow.