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.