Longitudinal waves are waves which move particles in the direction of the wave motion. They are the counterparts to transverse waves which move particles parallel to the direction of wave motion.
Let's talk about longitudinal waves, now that sounds like a really weird word. What on earth could I mean by that? Well longitudinal, along so while transverse means that the motion of the medium the disturbance is perpendicular to the direction of the wave motion. Longitudinal means that the direction of the medium is along the direction of the wave motion. So if the wave is going this way, the medium is going to go back and forth like that instead of up and down like that, or forward and backward like that. So that's the idea with the longitudinal wave. Now longitudinal waves are kind of difficult to write down and to draw but we can see one very easily with this slinky. So I have this guy Sidney here and now what I'm going to do is I'm going to generate a longitudinal pulse inside of the slinky. I'm going to do that by pushing this end back and forth and you can see here that the slinky itself is moving back and forth in this direction and the wave is also moving in that direction. So a transverse wave will look like that, whereas a longitudinal wave looks like that.
Now we have 2 different waves that we can do this just like we've got with transverse waves we could imagine displacing the medium up or displacing it down. With a longitudinal wave we could imagine making the medium more compact or making the medium less compact like that. Each one generates a longitudinal wave, now on tests and other types of reading assessments you will see representations of longitudinal waves and they'll look like one of these 2 diagrams kind of abysmal. So it takes a little while to understand what these diagrams are trying to represent. The lines in this top diagram are trying to show you where the material is, where the medium is. So in this top diagram the medium here is real close together. This is called a condensation, and you can kind of see why, the materials are condensed here.
On the other hand over here the material is spread out, so this is called a rarefaction okay so this type of diagram is actually not that difficult to read once you understand what it's trying to say. Same thing with this one, again it looks kind of lousy but what they're trying to get across is what we just saw with the slinky. They're to get across here we've got a rarefaction and right here we've got condensation. So that's the idea, not difficult at all so the example, well the major example of a longitudinal wave is sound. Sound is definitely a longitudinal wave and that's longitudinal waves.