Which type of wave has the medium move back and forth in the same direction the wave travels?

• A. Transverse wave
• B. Compressional (Longitudinal) wave
• C. Sound wave
• D. Surface wave

Answer: (B)

How the particles in the medium move relative to the direction the wave travels. In a longitudinal (compressional) wave, the particles oscillate back and forth along the same line as the wave’s travel. As the wave moves, regions of compression where particles are crowded alternate with regions of rarefaction where they’re spread apart, and these compressions propagate forward through the medium. That parallel motion—particle movement in the same direction as the wave—is what this description is capturing.

In a transverse wave, the particles move up and down (perpendicular to the direction of travel), so their motion isn’t in the same direction as the wave. Surface waves mix motions but aren’t simply back-and-forth along the travel direction, so they don’t fit the description as neatly. An everyday example of the longitudinal type is sound traveling through air, where the air particles push together and then pull apart along the path the sound is moving. This alignment of particle movement with the wave’s direction makes longitudinal the best fit.