longitudinal wave
Summary
A longitudinal wave is one in which particles of the medium oscillate parallel to the direction the wave travels, forming compressions and rarefactions. Sound is the key example, and it needs a medium, so it cannot travel through a vacuum.
Detail
Meaning: Particles of the medium oscillate along the same line the wave travels. They bunch into high-pressure compressions and spread into low-pressure rarefactions. The particles oscillate about equilibrium; the disturbance moves, not the matter. Sound in air and other fluids is longitudinal, whereas a transverse wave (light, waves on a string) has displacement perpendicular to travel.
Equation: v = fλ, where v is wave speed (m/s), f is frequency (Hz), and λ is wavelength (m), the distance between adjacent compressions.
On the MCAT: Questions ask you to identify sound as longitudinal, compare it with light, or predict speed changes between media. Sound is generally fastest in solids and slowest in gases. Frequency stays constant across a boundary; speed and wavelength change. On a pressure-versus-position graph, compressions are pressure maxima; on a displacement-versus-position graph, compressions and rarefactions occur where displacement is zero (pressure and displacement are 90° out of phase).
Common confusion: Students read the sine curve as the medium moving up and down. For sound, that curve plots pressure or displacement against position; the particles move back and forth along the travel direction. Longitudinal waves cannot be polarized, while transverse waves can.
Sources
- Physics (Giancoli)
- Physics (Cutnell and Johnson)
- Fundamentals of Physics (Halliday, Resnick, and Walker)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.