Now this brings us to the meet of our chapter which is really ask in the question what is sound. So if you want to think of sound as something that travels in the air and reaches your ears, you are right. Now we are going to go in to it deeply and ask the question what exactly is sound. So let’s begin. Let’s take a guitar and let’s pluck one of the string and observe what happens here. What you observe? What looks like that? What looks like really fast oscillation? If you want to slow things down right look at what you see- you begin o observe that the strings goes outwards and reaches a maximum point. And when it does that , what does it do? there is air all around this little string here in this case it goes that we press air together in other words there is a higher density of air in that little pocket. Next to the string as you can see- what you observe there is a high pressure, from your understanding of pressure from previous chapters you know that there is a higher pressure there. Now what happens to the string immediately? The string is gonna come back. When it does that what is what is really doing it’s a kind of pulling here behind it push some air and come back now which means that in this gap it is going to be lesser which low pressure so in one up and down, what the string is really doing is compress the air and is brought back and it is kept a little bit of air less for some time, so high pressure, low pressure. Now what is the string gonna do again it is gonna create this again. But even if it does not do that this high pressure area in other words air push in that way , what does it going to do? It is gonna try and push the air in other side of it even more. Right? So this compression is going to push little bit more in that side and that is going to little more in the other side this in other words compression is going to travel along with rarefaction following it. right? in other words you push some air that all these balls of air like each other become closely packed again the area again the next area zone as you can see. so in other words what begin to happen what is that this compression almost like that it starts moving through the air I am showing it in one line but you know that is happening all the circle around because air starts moving I all the possible direction. It starts getting compressed and rarefacted in all possible directions. So there is one compression and one rarefaction travelling all the possible directions. So now what we are going to do? So we are going to take the string of the guitar, visualize air parallel with this slinky as it compresses and come back. See both as a graph and watch what you observe. So the string of the guitar moves upwards, compresses the air, that equal to somebody pushing this slinky, pushing it that way and that is equal to somebody creating a pressure in the air and that is going to be presented with a little peak in the graph, as you can see. So the pressure peaks and now the strings of the guitar get pulled back in other words the pressure drops and this way the slinky is going to pull back again so the compression in the slinky is forward when the spring is going forward, but the back is little more bit elongated and that in the graph is represented as a down, a trough and a crest. So what does it really shown here? There is a pressure up and pressure down. And that’s way visualizing it, and the way in which it is drawn as graph. Then what is going to happen if you repeat it again. So, you got the string to move forward again, so one more compression, one more up. So, if you were let this run through time and that’s what you look through. Yet, the strength, compressing and rarefacting the air compressing pushing it back, compressing pushing it back, so on with the slinky and so on with the graph. The graph is a good way to visualize this. Rather is a very convenient way to visualize what is happening for us. And you can also begin to observe the graph is actually more suitable more… for the transfer of the ray, right. Because its exactly how if you have set up a way to do it, that’s how exactly it will look. Right? In this case it happens to be what we are observing happens to be what kind of waves-that’s right longitudinal wave, because the particles are pushing and they are compressing that way, so each particle are aware what’s it really doing. It’s just moving away from its position and coming back and oscillating back and forth about its position. This way the sound itself is moving in the same position, therefore, it’s a longitudinal way, so longitudinal waves are usually a bit to visualize therefore, the graph actually gives you more of a transverse representation, because if you have taken a string and done that’s how the graph will look it. But in this case that represents the sound and slinky- both of which are longitudinal waves.