2.1.9 Acousto-Optics: Velocity Of Sound In Liquids by Laser
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
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.
- 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.
- 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.
- 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).
- 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).
- G. W. Willard, ``Criteria for
Normal and Abnormal Ultrasonic Light Diffraction Effects,'' J. Acous. Soc. Amer. 21,
103 (1949). QC221.A4.
- 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.
- 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
- 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).
- 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.
- 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.
- E. Hecht, Optics, Second Edition, New York: Addison-Wesley
(1987), Section 10.2.7, p. 424, Section 14.1.1. p. 559.
- Data source: American Institute for
Physics Handbook, Third Edition New York:McGraw-Hill (1972), Section 3, Acoutics,
- Data source: W. Schaafs, Landolt-Börnstein
New Series Group II, Atomic and Molecular Physics, Volume 5, Molecular Acoustics,
New York: Springer-Verlag (1967).
- Uniphase Laser Specifications for
Model 100 and 1100 series.
- Acoustic Transducers information
and specs from Clevite.
- United Detector Planar-Diffused Silicon
Photodiodes spec sheets including the photodiode used for detection in this
- P. G. Witherell and M. E.
Faulhaber, "The Silicon Solar Cell as a Photometric Detector," Appl. Optics 9,
- R. W. Gammon, New sample cell
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