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What are waves?
What are waves?

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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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Transcript: Video 1  The propagation of a longitudinal wave along a spring. (0:12 min) (Longitudinal and transverse waves, 2012)

NARRATOR
In a longitudinal wave, the disturbance that makes up the wave is along the direction in which the wave travels. Longitudinal waves are also referred to as compression waves.
End transcript: Video 1  The propagation of a longitudinal wave along a spring. (0:12 min) (Longitudinal and transverse waves, 2012)
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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.

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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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An animation showing the propagation of a longitudinal wave as multiple black dots apparently moving from left to right. When looked at closely, however, individual dots are repeatedly moving backwards and forwards within a small horizontal window (about 5 millimetres in this diagram). Compressions occur when the dots coincide vertically and rarefactions occur when the dots are less well aligned. Compressions and rarefactions genuinely move across the screen from left to right, even though the particles within them do not. One compression has a black up arrow labelled ‘wave’ below it that follows it across the screen from left to the centre. Five individual dots have been coloured red to make it easier to see their individual back-and-forwards motion, and two of these dots are labelled with a red up arrow and the word ‘particle’ to exaggerate the motion further.

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...

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.

  • 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.