Which term describes a wave in which the medium moves back and forth in the same direction the wave travels, like sound waves?

• A. Transverse wave
• B. Longitudinal wave
• C. Compressional wave
• D. Wave

Answer: (C)

The key idea is how the medium’s motion relates to the direction the wave travels. In a longitudinal wave, the particles of the medium move back and forth along the same line as the wave is moving. This back-and-forth motion creates compressions (areas where particles are squeezed together) and rarefactions (areas where they are spread apart). Sound waves are a classic example of this behavior, since air molecules vibrate along the direction the sound wave travels, producing alternating high- and low-pressure regions.

The term most commonly used for this is longitudinal wave. Compressional wave describes the same phenomenon by highlighting the pressure changes, and it’s a valid descriptor in some contexts, but longitudinal wave is the standard label for this type of motion. By contrast, a transverse wave involves particle motion perpendicular to the direction of travel (like a vibrating string), and simply calling it a “wave” doesn’t specify this motion.