A wave is a disturbance that propagates through space carrying energy but not matter. This distinction is at the heart of everything that follows: when a wave passes through a medium, each little piece of the medium oscillates about its own equilibrium position and then returns to it, while what truly travels — moving even over great distances — is the disturbance, and with it the energy it carries. Waves are everywhere: sound, light, seismic waves, radio waves, and even ripples on the surface of water.
A mechanical wave is a disturbance that propagates in a material medium (a string, air, water, rock). Unlike electromagnetic waves, a mechanical wave needs a medium in which to exist: it cannot propagate in a vacuum. Mechanical waves are distinguished by the direction in which the medium oscillates relative to the direction in which the wave advances.
Principle — Transverse vs longitudinal
- Transverse wave: the disturbance is perpendicular to the direction of propagation. Examples: the wave on a taut string, electromagnetic waves.
- Longitudinal wave: the disturbance is parallel to the direction of propagation. Typical example: sound, made up of compressions and rarefactions of the air along the same direction in which the sound travels.
Polarisation
Transverse waves can be polarised: the disturbance oscillates in a single well-defined plane. Longitudinal waves, on the other hand, necessarily oscillate along the direction of propagation and therefore cannot be polarised. It is precisely this property that allows, for example, light to be recognised as a transverse wave.
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