Brewster angle light scattering from bubbles in water: Observations and potential applications to the acoustics of natural microbubbles

Abstract

Consider the reflection of light from a clean gas bubble in water such that the light is polarized with its electric field parallel to the scattering plane. The Fresnel reflection coefficient should vanish when the local angle of incidence i is at Brewster's angle iB = arctan (n−1) ≈ 36.9 deg, where n = 1.33 is the refractive index of water. When the scattering angle θ is close to that of the Brewster scattering angle, θB = (180 − 2iB) ≈ 106.2 deg, this Brewster effect should be strongly manifested in the scattering pattern [D. L. Kingsbury and P. L. Marston, Appl. Opt. 20, 2348–2350 (1981)]. In the present research the first laboratory observations described are of Brewster effects in the scattering patterns of bubbles rising through water having radii ≈ 70μm. There is a noticeable reduction in the visibility of fringes in the scattering pattern for scattering angles ≈θB. (The fringes arise from the interference of a farside ray with the reflected ray.) Microbubbles of natural origin can be important in ocean and bioacoustics and Brewster effects may be useful in the optical characterization of such bubbles including the discrimination of optical scattering patterns of bubbles from those of particles or cells. Brewster angle scattering may also be useful for characterization of the surfactant coating that is thought to occur on some microbubbles in nature. [Work supported by ONR.]