Babinet's principle in the Fresnel regime studied using ultrasound

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.