Points to Ponder

A wave is not the bulk movement of the medium itself. Wind, for example, involves air molecules physically travelling from one location to another. A sound wave in air, by contrast, involves only the back-and-forth oscillation of air layers — compressions and rarefactions — with no net transport of the air as a whole.

What travels from one point to another in a wave is energy, not matter. The particles of the medium merely oscillate about their mean positions; they do not make the journey along with the wave.

In a mechanical wave, energy is transferred because neighbouring particles of the medium are coupled by elastic forces. When one particle oscillates, it exerts a force on its neighbour, passing the oscillation — and hence the energy — along the medium.

Transverse waves require a medium that resists shearing, i.e., one with a non-zero shear modulus. This is why they can travel only through solids. Longitudinal waves, on the other hand, require only bulk modulus (resistance to volume change), so they can propagate through solids, liquids, and gases alike.

In a harmonic progressive wave of fixed frequency, every particle of the medium oscillates with the same amplitude, but at any given instant, different particles are at different points in their cycle — they have different phases. In a stationary wave, the situation is reversed: all particles between two consecutive nodes oscillate in phase (same phase at a given instant), but their amplitudes vary from zero at the nodes to a maximum at the antinodes. …