Which statement defines a longitudinal wave?

Which statement defines a longitudinal wave?

• A. The medium moves perpendicular to the direction of travel
• B. The highest point of a transverse wave
• C. The part of a longitudinal wave where the particles are close together
• D. A wave in which the vibration of the medium is parallel to the direction the wave travels

Answer: (D)

Longitudinal waves involve the medium’s particles moving back and forth along the same line as the wave is traveling. This back-and-forth motion creates compressions (where particles are crowded together) and rarefactions (where they are spread apart). The defining feature is that the vibration direction is parallel to the direction the wave moves. That is why the statement describing parallel vibration is the one that defines a longitudinal wave. Contrast that with transverse waves, where the medium moves perpendicular to the wave’s travel (like ripples on a string or light waves). The mention of a crest points to a transverse wave’s peak, not a longitudinal wave. And while a compression is part of a longitudinal wave, simply naming a compressed region doesn’t define the wave type. The key idea is the parallel motion of the medium to the direction of travel.

Longitudinal waves involve the medium’s particles moving back and forth along the same line as the wave is traveling. This back-and-forth motion creates compressions (where particles are crowded together) and rarefactions (where they are spread apart). The defining feature is that the vibration direction is parallel to the direction the wave moves. That is why the statement describing parallel vibration is the one that defines a longitudinal wave.

Contrast that with transverse waves, where the medium moves perpendicular to the wave’s travel (like ripples on a string or light waves). The mention of a crest points to a transverse wave’s peak, not a longitudinal wave. And while a compression is part of a longitudinal wave, simply naming a compressed region doesn’t define the wave type. The key idea is the parallel motion of the medium to the direction of travel.