CRITICAL ANGLE SCATTERING BY A BUBBLE - PHYSICAL-OPTICS APPROXIMATION AND OBSERVATIONS

The intensity of light scattered by an air bubble in water is predicted by geometric optics to have a divergent angular derivative as the critical scattering angle phi //c is approached. Effects of diffraction in the angular region near phi //c are described. The Fraunhofer diffraction for scattering angles phi less than equivalent to phi //c is estimated using a simplified physical-optics approximation. A ringing and decay of the far-field intensity is predicted that is formally similar to the near-field diffraction of a straight edge. Observation of millimeter radius bubbles in water with collimated monochromatic illumination verifies the existence of this ringing. Other results are also presented. Implications for the critical angle scattering of white light are discussed.