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Observation of Brewster Angle Light Scattering from Air Bubbles Rising in Water

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Technical rept. no. 7, 15 Jul 1987-15 Aug 1988

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Air bubbles rising freely through distilled water were illuminated by laser light with incident polarization of the electric field parallel to the plane of incidence. The radii a of the bubbles investigated were from 50 to 100 micrometers. At Brewsters angle of incidence i sub B arctan m 36.9, the Fresnel reflection coefficient vanishes, where m is the relative index of refraction for a water-air interface, m 0.75. Therefore a minimum in scattered intensity should be observed at the Brewster scattering angle Theta sub B 180 deg - 2i sub B 106.2 deg. Furthermore interference patterns caused by different classes of rays should change drastically for angles around Brewster scattering angle Theta sub b. Experiments were done to detect these two phenomena by photographing the scattering light. The scattered intensity as a function of angle was inferred by scanning the photographs with a microdensitometer. To confirm the experimental results, they were compared with Mie theory calculation for scattering from spherical bubbles. Features of the scattering pattern due to ray interference could be understood using geometrical optics. The experiment confirmed that the observed features of the scattering pattern near Theta sub B are those predicted of a bubble having a clean surface. Since it has been previously calculated that the scattering pattern would be significantly different near Theta sub B if the surface of the bubble is coated by contamination, it is plausible that such a coating could be characterized optically. It is thought that the acoustics and dynamics of microbubbles in the ocean, or in other environments, may be affected by such coatings. Theses.

Subject Categories:

  • Physical and Dynamic Oceanography
  • Acoustics
  • Optics

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