Abstract

It is well known that large spherical drops of water in air give enhanced scattering at angles close to the rainbow angle θ_r ≃ 138°. Near θ_r, diffraction provides an essential correction to geometric optics and the scattering is a simple example of a fold diffraction catastrophe.¹ We observed² the scattering from spheroidal drops into the rainbow region and we find other diffraction catastrophes can be produced depending on the drop's axis ratio q = D/H. Here the drop's diameter is D in the equatorial plane and it is H along the vertical axis of rotational symmetry. An acoustic standing wave was used to levitate the drop and to control q. The drops were oblate (q > 1 ) and they were horizontally illuminated. With q ≃ 1.31, the observed scattering pattern corresponds to the focal section of a hyperbolic-umbilic diffraction catastrophe. For q differing somewhat from this value, the pattern was observed to separate into fold and cusp diffraction catastrophes. These novel manifestations of far-field diffraction catastrophes are applicable to inverse problems.

© 1985 Optical Society of America