Effect of microparticles on acousto-optic diffraction in water

In Raman-Nath diffraction by microparticles in water, even-order diffraction is enhanced selectively by the suspension of microparticles. Intensities of diffracted beams increase markedly for particles larger than 10 mu m in diameter. By microscopic observation, particles 10-20 mu m in diameter align periodically at the nodes of an ultrasonic standing wave. On the other hand, particles 1-7 mu m in diameter distribute widely around the nodes. Since larger particles tend to remain at the nodes even after the ultrasonic wave is removed, even-order beams remain diffracted for several seconds.