When the particle motion is parallel to the direction of wave propagation, the wave is classified as?

The classification of waves based on particle motion in relation to wave propagation is a fundamental aspect of wave mechanics. In a longitudinal wave, the particles of the medium move parallel to the direction of wave propagation. This means that as the wave travels through a medium, such as sound waves traveling through air, the individual particles oscillate back and forth in the same direction as the wave moves.

This characteristic allows longitudinal waves to effectively transmit energy through compressions and rarefactions, where areas of high pressure (compressions) are followed by areas of low pressure (rarefactions). This motion is distinct and critical for understanding wave behavior in various applications, particularly in the context of ultrasonic testing within Nondestructive Testing (NDT).

In contrast, transversal waves exhibit particle motion that is perpendicular to the wave direction, which is not applicable here. Surface waves, including Rayleigh waves, are combinations of longitudinal and transversal motion and occur at the surface of a medium; therefore, they do not fit the criteria stated in the question.

Thus, the choice that aligns accurately with the definition of waves where particle motion is parallel to wave propagation is the longitudinal wave.