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INDIANA UNIVERSITY, DEPARTMENT OF PHYSICS, P309 LABORATORY Laboratory #31: Debye-Sears: Speed of Sound in Water Goal: study the diffraction of light by density fluctuations in a liquid; determine the speed of sound by measuring the wavelength of ultrasound. Measure the compressibility of water Equipment: Cuvette, ultrasonic generator, laser diode, system of mirrors, frequency counter, thermometer. A. Physics: the Debye-Sears Effect A.1. Interaction between light and sound waves in a liquid A sound wave is characterized by regions of density that are higher or lower than that of the undisturbed medium M . These regions alternate in the direction of propagation, and are separated by the wavelength λ S of the sound in the medium. If f S is the frequency of the sound, the speed of sound, v S , is given by S S S f v λ = (1) In a transparent liquid medium, variations in density correspond to a deviation δ n M of the index of refraction from its average value n M . A light ray that travels perpendicular to the sound direction is refracted. The angle of refraction is zero if the ray coincides with a density maximum or minimum, and largest if the ray is halfway between. The range of refraction angles depends on the sound intensity. A.2. Diffraction by a phase grating fracted by an angle Note, that the wavelengths of the ght in vacuum, The transmitted light rays that are re α are enhanced if the path difference of rays originating from two points, separated by a sound wave length,
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