  The diffraction of ultrasound by a circular disk and an aperture of the same
size have been investigated as a demonstration of Babinet's principle in the
Fresnel regime. The amplitude and the phase of diffracted ultrasonic waves have
been measured, and a graphical treatment of the results is performed by simply
drawing vectors (phasors) in the complex plane. The results verify Babinet's
principle. It is also found that the incident wave is indeed 90 deg behind the
phase of the wave passing through on the central axis of a circular aperture.
This paradox has previously been regarded as a defect of Fresnel's theory. The
apparatus used is intended as a table-top instrument for the student laboratory
in general science and engineering classes.
