2.1.9 Acousto-Optics: Velocity Of Sound In Liquids by Laser Diffraction

The objective of this experiment is to study the propagation of ultrasound in liquids and the interaction of ultrasound with light. By high-frequency RF the piezoelectric oscillates mechanically, exciting longitudinal (compression) waves in the liquid. These waves cause regions of varying density which constitute an optical grating. It would be a good idea to review the subject of optical gratings in an optics book of your choice. On is suggested below. From the spacing of the maxima in the diffraction pattern produced by such a grating, the wavelength of the ultrasound in the liquid can be determined. The intensities of the maxima in the diffraction pattern can also be studied. The power emitted by the piezoelectric crystal, the frequency of the ultrasound, the type of liquid, the type of ultrasound wave (traveling or standing), the alignment of the system, and the distance from the crystal at which the light interacts with the ultrasound are variables which are likely to affect the quantitative diffraction pattern.

As always, the student should investigate the effects of as many variables as possible, given the time constraints of the course. A common starting point is to measure the velocity of ultrasound in a few different liquids and liquid mixtures, and compare the result with theories of sound propagation in fluids. (see the Burton reference below). For intensity measurements, the photocell might need to be checked for linearity and perhaps calibrated with a photometer and/or a good pair of polarizers. A program is available which fits the peak intensities to the theoretical prediction of Raman and Nath (see the reference below). Careful! RF (radio frequencies) can shock or burn.

For safety we would prefer that you work with only the following fluids in this experiment: Water, Acetone, Methanol, Ethanol, n-Propanol, Cyclohexane, Toluene, n-Octane, and Ethylene Glycol. The following mixtures can also be studied: (Methanol and water) and (Ethylene Glycol and water). Several of these are flammable and have some toxicity so that one should avoid open containers and skin contact. All have sufficient data in the references below ( 12. and 13.) to allow comparison with your measurements of velocity and attenutaion, corrected for temperature.


  1. P. Debye and F. W. Sears, ``On the Scattering of Light by Supersonic Waves'', Proc. Natl. Acad. Sci. U.S.A. 18, 409 (1932). This is the original report about this effect. 
  2. C. V. Raman and N. S. N. Nath, Proc. Indian Acad. Sci. A2, 406 (1935); A2, 413 (1935); A3, 75 (1936). There were more papers in this series. (See Pierce et al. reference below) for a more complete list. This is the theory for the diffraction including cases where the phase modulation is large, a region which can be reached with Grad Lab equipment. Q73.I6 Ser. A.
  3. D. T. Pierce and R. L. Byer, ``Experiments on the Interaction of Light and Sound for the Advanced Laboratory,'' Am. J. Phys. 41, 314 (1973).
  4. S. Chadda and S. P. Mallikarjun Rao, ``Determination of Ultrasonic Velocity in Liquids Using Optical Diffraction By Short Acoustic Pulses,'' Am. J. Phys. 47, 464 (1979).
  5. G. W. Willard, ``Criteria for Normal and Abnormal Ultrasonic Light Diffraction Effects,'' J. Acous. Soc. Amer. 21, 103 (1949). QC221.A4.
  6. J. L. Hunter, ``The Absorption of Ultrasonic Waves in Highly Viscous Liquids,'' J. Acoust. Soc. Am. 13, 36 (1941). This is background for possible measurements with our newer diffraction technique. 
  7. C. J. Burton, ``A Study of Ultrasonic Velocity and Absorption in Liquid Mixtures,'' J. Acous. Soc. Amer. 20, 186 (1948). This give data for comparison to velocities which can be done with our newer diffraction technique. QC221.A4
  8. F. E. Fox and G. D. Rock, ``An Ultrasonic Source of Improved Design: Optical Studies of Ultrasonic Waves in Liquids,'' Rev. Sci. Instrum. 9, 341 (1938).
  9. L. N. Bergman, Ultrasonics and Their Scientific and Technical Applications, London: G. Bell and Sons Ltd. (1938). This is a pre-laser book which covers beautiful pdf/acousto-optic techniques. QC243.B42.
  10. W. G. Cady, Piezoelectricity: an Introduction to the Theory and Applications of Electromechanical Phenomena in Crystals, New York: Dover (1964).  Missing from Grad Lab library, #9. QC585.C3.
  11. E. Hecht, Optics, Second Edition, New York: Addison-Wesley (1987), Section 10.2.7, p. 424, Section 14.1.1. p. 559.
  12. Data source: American Institute for Physics Handbook, Third Edition New York:McGraw-Hill (1972), Section 3, Acoutics, p.3-86.
  13. Data source: W. Schaafs, Landolt-Börnstein New Series Group II, Atomic and Molecular Physics, Volume 5, Molecular Acoustics, New York: Springer-Verlag (1967).
  14. Uniphase Laser Specifications for   Model 100 and 1100 series.
  15. Acoustic Transducers information and specs from Clevite.
  16. United Detector Planar-Diffused Silicon Photodiodes spec sheets including the photodiode used for detection in this experiment.
  17. P. G. Witherell and M. E. Faulhaber, "The Silicon Solar Cell as a Photometric Detector," Appl. Optics 9, 73 (1969).
  18. R. W. Gammon, New sample cell configuration (1998).


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