Abstract
If the optical wave front is not normal to one of the principal axes of the indicatrix, diffraction of light in crystals, caused by ultrasound, is observed with maximum intensity when the incident beam is off the Bragg angle by an amount depending on double refraction. In a crystal plate of finite dimensions a standing ultrasonic wave also generates waves with propagation directions which are inclined to the normal of the plate, and interfere with the measurement of sound velocities by the improved Schaefer–Bergmann method. These difficulties are overcome by an appropriate choice of the angle of the incident light beam. Experiments with triclinic and trigonal crystals are reported. Formulae are derived for calculating the angles of incidence necessary for any measurements of sound velocities in crystals.
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