Longitudinal waves, which are sometimes also called compressional waves, are waves in which the
particles travel in the same or opposite direction as the waves propagation, so the waves travel.
Some examples of longitudinal waves are sound waves, which use the vibration of air or water in
order for the sound to travel; particle movement in an elastic medium, so things like an elastic
band or a spring because of the compressional component and then the elastic part of that
material they can produce longitudinal waves or act in the same way; and seismic P waves which
we see created by earthquakes or large explosions in the earth. So the way in which those waves move
through the solid component of the earth through seismic P waves which are longitudinal waves.
So this slinky is going to help us to look at what a compressional or longitudinal wave looks like,
or how it behaves. A compressional or longitudinal wave is one in which the medium moves in the same
direction or opposite direction to the waves travel. So here, this slinky is behaving like
our wave. So if we compress some of it and let it go then the wave's travel is going to
be that way. Okay, but I want you to watch the ribbon that I've tied on here and I want you to
see what the direction of the ribbon does and see how it's behaving in relation to the wave.
So you can see that the ribbon is moving either in the same direction or opposite direction as
the movement or the direction of the travel of the wave. So this is how the particles
in a compressional wave work; they moves with or against in the opposite direction. This is
different to your transverse wave, so remember the transverse waves - the particles move at
right angles. So you know transverse wave, if we're looking at transverse wave like this,
the particles are moving up and down even though the direction of the wave is moving that way.
I'd like to show you how to label the features of a longitudinal wave. There are some features
that are the same between a transverse and a longitudinal wave and there's some unique
features just for longitudinal waves as well. So let's have a look at this top diagram. There's a
few different ways that longitudinal waves can be represented in diagrams and we'll start with
this one first because this one looks like the slinky that we were demonstrating with
before. If we look at how the slinky or how this particular diagram is represented you
can see here that the particles are really close together, as you move further along
to the right here they are more spread apart and then they come back together again over
here. And so when they are compressed like this we call those sections compression.
So we've got the same thing here and here they're both compressions
and in the middle where they're more spaced apart we call that rarefaction.
Now from one compression to the next compression or from one point on the longitudinal wave to the
same point on the next longitudinal wave we call that the wavelength. So the definition is the same
between the transverse and the longitudinal wave it just looks different on the diagram.
And remembering that in a longitudinal wave the displacement of the particles are going
in the same or opposite direction to the waves travel. So this is the displacement.
In this case the displacement of the air particles, or whatever the medium is that
the wave is being transferred through. So let's have a look at another diagram. So this one at the
bottom of the page there it does look a little bit different to the top one but again we can
still see those main features. So this time we're representing our longitudinal wave with lines,
and here we can see that there is the compression section where the lines are really close together.
So I'm going to label that as the compression. Here, where the lines are really spaced apart,
we call that rarefaction - the same as in the above diagram. And again,
that the wavelength is from one compression to the next compression. And this area here is showing us
the displacement or the movement of the particles in relation to the direction of the travel.
And one last diagram, the other way we can see it represented is with dots. So again if we compare
it with the other two diagrams we can still see that these sections where there are a lot of dots
all close together and that is where we still call that the compression. So this part here,
compression. And where they're further apart like we can see here and here and here,
that is called the rarefaction. And just like all the others,
if we go from one compression to the next that's our wavelength. So each of
these three types of diagrams show us our longitudinal wave just in different ways.