Longitudinal waves

In a longitudinal wave, particles of the medium oscillate back and forth parallel to the direction in which the wave transfers energy. Each particle oscillates about its equilibrium position rather than travelling along with the wave.

As particles move, they affect neighbouring particles, passing the disturbance and its energy through the medium. The wave can therefore transfer energy without the medium as a whole travelling with it.

Sound in air

Sound in air is a longitudinal wave. As air particles oscillate, they produce alternating regions called compressions and rarefactions:

• In a compression, particles are closer together, so particle density and air pressure are higher than their equilibrium values.
• In a rarefaction, particles are farther apart, so particle density and air pressure are lower than their equilibrium values.

These regions travel through the air as the sound wave transfers energy, while the air particles oscillate back and forth about their equilibrium positions.

Transverse waves

A transverse wave transfers energy in a direction perpendicular to the direction of its oscillations. Perpendicular means that the two directions are at right angles to each other.

Transverse waves on a string

When a transverse wave travels along a string, each particle of the string oscillates about its equilibrium position: its position when the string is undisturbed. For example, if the wave travels horizontally along the string, the particles move up and down.

The wave and its energy travel along the string, but the particles do not travel along with the wave. They oscillate perpendicular to its direction of travel.

Electromagnetic waves are transverse

In an electromagnetic wave, electric and magnetic fields oscillate. Both fields oscillate perpendicular to the direction in which the wave propagates, and they are also perpendicular to each other.

All electromagnetic waves travel at the same speed in a vacuum: approximately $$3.0 \times 10^8\ \mathrm{m,s^{-1}}$$.

Wave polarisation

A transverse wave oscillates perpendicular to its direction of travel. There is more than one possible direction for these oscillations: they can point in different directions in the plane perpendicular to the wave’s travel.

When the oscillations are restricted to one direction in that plane, the wave is polarised. Polarisation changes the direction of oscillation, not the direction in which the wave travels.

A longitudinal wave oscillates parallel to its direction of travel, so it has no sideways oscillation direction to restrict in this way. It cannot be polarised. Therefore, if a wave can be polarised, it must be transverse.

Polarisers and electromagnetic waves

A polariser selectively transmits the part of a transverse wave’s oscillation aligned with its transmission direction. For example, if a wave’s oscillation is at an angle to that direction, only its component along the transmission direction is transmitted. The transmitted wave is polarised in that direction.

Polaroid material acts as a polariser for electromagnetic waves. It transmits the component of the wave’s electric field along its transmission direction. The transmitted electric field therefore oscillates along that direction, perpendicular to the wave’s travel.

Polarisation and aerials

A transmitting aerial produces a polarised electromagnetic wave. A receiving aerial responds most strongly when it is aligned with the direction in which the wave’s electric field oscillates.

Reception is therefore maximised by aligning the transmitting and receiving aerials so that the receiving aerial is parallel to the electric field. If the receiving aerial is perpendicular to that field, reception is greatly reduced.