Problem 5.132

Figure 5.27 illustrates an arrangement employed in observations of diffraction of light by ultrasound. A plane light wave with wavelength $\lambda=0.55 \mu \mathrm{m}$ passes through the water-filled tank $T$ in which a standing ultrasonic wave is sustained at a frequency $v=4.7 \mathrm{MHz}$. As a result of diffraction of light by the optically inhomogeneous periodic structure a diffraction spectrum can be observed in the focal plane of the objective $O$ with focal length $f=35 \mathrm{~cm} .$ The separation between neighbouring maxima is $\Delta x=$ $=0.60 \mathrm{~mm} .$ Find the propagation velocity of ultrasonic oscillations in water.