What are waves?
◄Previous: 3.1 Transverse waves

# 3.2 Longitudinal waves

In a longitudinal wave the direction of motion of the particles is in the same direction as the wave travels. This can be seen using a long spring called a slinky.

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Video 1 The propagation of a longitudinal wave along a spring. (0:12 min) (Longitudinal and transverse waves, 2012)
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Figure 4 shows how the compressions and rarefactions in a longitudinal wave are similar to the peaks and troughs of transverse waves, but instead of the peaks and troughs being at a distance from an undisturbed level, they are where the particles are pushed together, and apart.

Figure 4 The propagation of a longitudinal wave showing the passage of wave fronts that are comparable to the peaks in Figure 3. The red arrows and dots indicate four particles so that their motion can be more easily observed.
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Sound travels as a longitudinal wave. Longitudinal sound waves travel in the atmosphere through the compressions and rarefactions of atmospheric particles.

Give a reason why sound waves cannot propagate in space.

## Answer

Sound waves cannot propagate in space because there are no particles to allow the wave to propagate.

In Figure 2, we visually identified two properties of the wave: its frequency and its amplitude. These can be better illustrated on a graph.

◄Contents►Next: 4 Graphical representation of waves

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