Criteria for Normal and Abnormal Ultrasonic Light Diffraction Effects

The normal or common type of ultrasonic light diffraction occurs when traveling sound waves act like a phase grating. The spacing and intensities of the respective orders of spectra are given by the Raman-Nath theory. Experimentally and theoretically, this type of diffraction is obtained when the product of (width of sound beam) × (frequency)2 × (sound amplitude) is sufficiently low. When the light beam is maintained parallel to the sound wave fronts, the right and left spectra of the same order are of equal intensity. An abnormal or uncommon type of ultrasonic light diffraction occurs when the sound waves diffract the light in a manner satisfying the Bragg reflection law, well known in x-rays. The spacing of the spectra is essentially the same as before but right and left spectra do not appear simultaneously. Strong intensity in a given portion of the spectra is obtained only when the glancing angles of incidence φ and of reflection θ are equal, as in mirror reflection, and when λ, Λ, and θ satisfy the Bragg law nλ = 2Λ sinθ, (λ and Λ are the light and sound wave-lengths, respectively, and n is the order of spectra). This type of diffraction is obtained only when the single product (width of sound beam) × (frequency)2 is sufficiently large. Experiments and theory indicate the above criteria for these two types of light diffraction. Theory also indicates the possibility of other types of abnormal diffraction, such as mirror reflection, diffraction satisfying ruled grating theory, nλ = Λ(sinφ + sinθ), where φ does not necessarily equal θ, and others.