Figure 5.27 illustrates an arrangement employed in observations of diffraction of light by ultrasound. A plane light wave with wavelength ? = 0.55Âľm passes through the water-filled tank T in which a standing ultrasonic wave is sustained at a frequency v = 4.7 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 cm. The separation between neighbouring maxima is ?x = 0.60 mm. Find the propagation velocity of ultrasonic oscillations in water.