Transverse Waves and Longitudinal Waves

Progressive (travelling) waves come in two kinds, distinguished by how the medium's particles vibrate relative to the direction the wave itself is moving.

Transverse waves. A wave is transverse when the particles of the medium vibrate PERPENDICULAR to the direction the wave travels. Water waves are a familiar (if slightly idealised) example, since the water molecules bob up and down while the wave pattern itself moves horizontally along the surface. Characteristics: (1) every particle in the wave's path vibrates perpendicular to the propagation direction, with the same period and amplitude; (2) as the wave passes, the medium divides into alternating crests (regions of positive/maximum displacement) and troughs (regions of negative/minimum displacement); (3) a crest plus its adjacent trough together form one full cycle, and the distance between any two consecutive points in the same phase (crest-to-crest, say) is the wavelength; (4) it is the advancing crests and troughs that carry the wave's energy forward; (5) transverse waves can only travel through solids and along the surface of liquids -- they CANNOT travel through the bulk of liquids or through gases, because those lack the shear elasticity (elasticity of shape) a transverse wave needs (electromagnetic waves are the one transverse-wave exception that needs no medium at all); (6) as a transverse wave advances, the medium's pressure and density at any point stay unchanged -- only its SHAPE changes periodically; (7) if every particle's vibration is confined to a single plane, the wave is said to be polarised -- transverse waves CAN be polarised; (8) the medium must possess elasticity of shape to carry a transverse wave. …