  A simple experimental technique for measuring the phase and amplitude of
diffracting ultrasound wave [A. Hitachi and M. Takata, Am. J. Phys. 78, 678
(2010)] has been applied to diffracting objects with straight edges as a
demonstration of the Cornu spiral. Babinet's principle is studied observing the
ultrasound field behind a slit and a complementary strip obstacle and is
verified directly by comparing vectors (phasors) in the complex plane. The
phase of the diffracted wave observed in the geometrical shadow of the straight
screen has the form of a cylindrical wave originating at the edge of the
straight screen as the boundary diffraction wave proposed by Young. In
addition, the incident wave has a phase delay of pi/4 behind the wave passing
through on the center line of the slit, the plane of symmetry, has been
observed as predicted by Huygens-Fresnel diffraction theory.
