longitudinal

Meaning: A longitudinal wave is one in which the particles of the medium oscillate parallel to the direction the wave travels. The disturbance appears as compressions (particles crowded, higher pressure) and rarefactions (particles spread out, lower pressure), and the wavelength is the distance between adjacent compressions. Sound in air and other fluids is the standard example. A transverse wave differs: displacement is perpendicular to travel, as in a wave on a string or in light.

Equation: v = fλ, where v is wave speed (m/s), f is frequency (Hz), and λ is wavelength (m). For sound in a fluid, v = √(B/ρ), where B is the bulk modulus (Pa) and ρ is density (kg/m³).

On the MCAT: Passages on sound ask you to name the wave type, compare speeds in different media, or link pressure to displacement: in a standing wave, a displacement node is a pressure antinode. Only transverse waves can be polarized.

Common confusion: Students picture air molecules traveling from the source to the ear. Each particle only oscillates about its equilibrium position; the pattern and energy move, not the matter. Sound in fluids is longitudinal, but solids can also carry transverse waves.