In which medium can ultrasonic vibrations propagate only in the longitudinal mode?

Ultrasonic vibrations can propagate in different modes depending on the medium they traverse. The correct answer, which refers to the medium where ultrasonic vibrations propagate solely in the longitudinal mode, is indeed liquid.

In a liquid, ultrasonic waves travel by compressing and rarefying the liquid in the same direction as the wave propagation, which is characteristic of longitudinal waves. This behavior occurs because liquids lack a structure that allows for shear stress, which is necessary for the propagation of transverse (shear) waves. In this case, particles in the liquid move in the same direction as the wave, creating regions of high and low pressure.

In contrast, while solids can support both longitudinal and transverse waves due to their ability to withstand shear stress, and gases can also transmit longitudinal waves, they have different characteristics compared to liquids. A vacuum, while being the absence of matter, cannot support any wave propagation, including longitudinal waves.

Understanding these distinctions about wave propagation in different media is crucial for applications in nondestructive testing, as it affects the choice of technique based on the material being evaluated.