Click here to purchase the entire book in PDF format. Longitudinal vs. Transverse Waves There are basically three types of waves used to transmit energy through a medium or substance. Transverse Longitudinal Torsional We're only really concerned with the first two. Transverse waves are the kind we see every day in ropes and puddles. They're the kind where the motion of the particles is perpendicular to the direction of the wave propagation as can be seen in Figure 3.5. What does this mean? It's easy to see if we go fishing... A boat on the surface of the ocean will sit there bobbing up and down as the waves roll past it. The waves are travelling towards the shore along the surface of the water, but the water itself only moves up and down, not sideways (we know this because the boat would move sideways as well if the water was doing so...) So, as the water molecules move vertically, the wave propagates horizontally. Figure 3.5: A snapshot of a transverse wave on a string. Think of the wave as moving from left to right - but remember that the string is really only moving up and down. Longitudinal waves are a little tougher to see. They involve the compression (bunching together) and refraction (pulling apart) of the particles in the medium such that the motion of the particles is parallel with the direction of propagation of the wave. The easiest way to see a longitudinal wave is to stretch out a Slinky between two people, squeeze together a small section of it and let go. The compressed part will appear to move back and forth bouncing between the two ends of the spring. This is essentially the way sound travels through air particles. Torsional waves don't apply to anything we're doing in this book, but they're wave in which the particles rotate around the axis along which the wave propagates (like a twisting rod). This type of wave can be seen on a Shive wave machine at physics demonstrations and science and technology museums.