Strobo-photoelastic visualization of Lamb waves with negative group velocity propagating on a glass plate

An elastic wave with negative group velocity transports wave energy in the direction opposite to the phase velocity. This phenomenon has been predicted in certain modes of Lamb waves. This is still difficult to believe, despite the fact that some experimental evidence has been reported so far. In this paper, direct evidence of the negative group velocity is presented which was obtained by means of visualization of pulsed Lamb waves propagating on a glass plate in A2 mode.