Longitudinal waves show areas of compression and rarefaction.

The idea

In a longitudinal wave the particles oscillate backwards and forwards along the same line that the wave travels, so they bunch up in some places and spread out in others. A region where the particles are pushed closer together, and the pressure is higher than normal, is a compression. A region where they are further apart, with lower than normal pressure, is a rarefaction. The pattern moves forward while each particle stays around its own average position. The wavelength is the distance from one compression to the next compression, or from one rarefaction to the next, which is why a diagram must show at least two of them before it can be measured. In air a compression is a region of slightly raised pressure, and it is that pressure variation which a microphone or an eardrum detects.

What earns the marks

• A compression being a region where the particles are closer together, or the pressure is higher.
• A rarefaction being a region where the particles are further apart, or the pressure is lower.
• The wavelength measured from one compression to the next.
• The particles oscillating parallel to the direction of energy transfer.

Common mistakes and how to put them right

• Saying the particles travel from the source all the way to the detector.

• Labelling a compression as a crest and a rarefaction as a trough.

• Measuring the wavelength from a compression to the neighbouring rarefaction, which is half a wavelength.

• Describing a rarefaction as a vacuum rather than as a lower pressure region.

Exam tips

• When labelling a longitudinal diagram, point your label lines at the centre of the bunched or spread region.
• Mention pressure as well as particle spacing; either wording can earn the mark.
• Remember that each particle only oscillates about a fixed position, and say so.