Focal shift of convergent ultrasonic beams reflected from a liquid–solid interface

It is shown by schlieren photography that a convergent acoustic beam reflected from a liquid–solid interface can exhibit a focal shift in the axial direction when the angle of incidence deviates slightly from the critical angle. The results confirm the recent predictions of Bertoni et al. [Trait. Sign. 2, 201–205 (1985)] about this interesting detail of the general Rayleigh angle phenomenon.