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. The results verify Babinet‟s principle. It is also found that the wave without the diffracting objects is indeed 90 deg in phase behind the wave from the center of the zone system. This paradox has previously been regarded as a defect of Fresnel‟s theory.