 One of the things that we do in class is take out some springs, start making some pulses, and seeing what wave behavior is. So what kind of things can a wave do? We're going to take a look at two different things in this video. We're going to look at reflection and the superposition of standing waves. Imagine you throw a pebble into a pond. You would see all of these ripples, all of these waves. These waves, when we're looking at it from above, are called a wave front diagram. If we were to take a ruler and draw an arrow at a right angle to all of the waves, we would call that arrow a ray. It's an easy way to figure out what's happening overall with the waves. Say that arrow hits a boundary. It would reflect. We call this phenomenon reflection, and it's governed by the law of reflection, which says the angle of incidence measured from this vertical line called the normal equals the angle of reflection. All types of waves reflect when they bounce off of something hard. Think of a sound wave hitting a hard surface. You hear an echo, but there's more than just a change of direction when the wave is reflected. Here's a pulse that's a crest. As it hits the boundary, it reflects back, but it inverts, it flips around in terms into a trough. But what would happen if we hit one of those returning troughs with another crest? Well then those two waves would overlap. This is called superposition, and in this case these two waves would cancel out. When we get waves to cancel out like that, we call it destructive interference. Now if the pulse reflects off a non-fixed boundary, like the end of a string that isn't tied down, it'll reflect back again, but it won't invert. If it starts off as a crest, it'll come back as a crest. So what's going to happen if I run one crest into another crest in the overlap? We'll get superposition again, but this time the two amplitudes will add together, and the resulting wave will be twice as tall as the last wave. This is called constructive interference. If we keep running these waves into each other over and over again, we get a pattern called a standing wave. In the standing wave we have continuous constructive and destructive interference, and it ended up giving us some unique positions in the wave. We have a spot in the wave where the wave doesn't actually move any. A spot where there's no amplitude. These are called nodes. We also have spots in the wave where this amplitude is its greatest. These are called the anti-nodes. You can see this phenomenon really well in this awesome animation by Walter Fent. I'll include a link down in the description. You can also see constructive and destructive interference in a wavefront model. See where the crests, the blue parts overlap with the troughs, the black parts of this wave. Those waves cancel out. They get kind of fuzzy. That's the destructive interference. And areas where the waves stay nice and clear and sharp are the constructive interference. For more information on this topic, check out my website at LDindustries.ca.