Q.Define a longitudinal wave.

[!TLDR] A longitudinal wave is one where the medium's particles vibrate parallel to the direction the wave travels, forming alternating compressions and rarefactions. [!ANSWER] A …

A longitudinal wave is one in which the particles of the medium, along the wave's path, vibrate in a direction PARALLEL to the direction the wave itself propagates. As it advances, the medium divides into alternating compressions (crowded, high-pressure regions where particles bunch together) and rarefactions (spread-out, low-pressure regions); a compression together with its adjacent rarefaction forms one complete cycle. Longitudinal waves require the medium to have elasticity of VOLUME, so -- unlike transverse waves -- they can travel through solids, liquids AND gases, though never through a vacuum. Because their vibration direction already coincides with the propagation direction, they cannot be confined …

State the defining relationship (parallel vibration vs propagation) directly, illustrated with the compression …

Mixing up 'parallel' and 'perpendicular' between the transverse and longitudinal definitions -- a very common slip whe …