In a longitudinal wave, which area corresponds to particles that are spread out?

• A. Compression
• B. Density
• C. Rarefaction
• D. Medium

Answer: (C)

In a longitudinal wave, energy moves as particles in the medium oscillate back and forth in the same direction as the wave travels. As the wave passes, some regions become crowded with particles—these are compressions. In between, the particles spread out, creating regions where there are fewer particles in the same space. That spread-out region is called a rarefaction.

Rarefaction describes the area where particles are spread out, leading to lower density and pressure compared to the compression regions. Compression, by contrast, is where particles are packed together more tightly. The term density refers to how much matter is in a given volume and isn’t the name of a specific region, and the medium is simply the substance the wave travels through.