7.2 Transverse and longitudinal waves

In a transverse wave, particles oscillate perpendicular to propagation; in a longitudinal wave, they oscillate parallel, producing compressions and rarefactions.

Classify the wave from particle motion, not from whether it is visible or mechanical. Polarisation is possible only for transverse waves.

A rope pulse is transverse; sound in air is longitudinal because air pressure changes travel through compressions.

Longitudinal does not mean “slow”, and transverse does not mean “up-down” in every orientation.

A displacement–distance graph shows the shape at one instant and a displacement–time graph shows one point’s oscillation; wavelength and period must be read from the correct axis.

Measure crest-to-crest or equivalent phase spacing for λ, and cycle time for T. Do not mix a spatial graph with a time graph.

A displacement–distance graph with 0.80 m between adjacent crests has λ=0.80 m; a time graph with 0.20 s per cycle has f=5 Hz.

Amplitude is read vertically on both graphs, but horizontal scale represents distance in one and time in the other.