So if this hole has a certain width w, how far apart are these? These are not w apart. We have to be careful because this hole has a certain width. What would d be in this case? Now we have to be pretty careful. This screen had to be very far away compared to the width of the hole. Remember, that relationship was d sin theta equals the path And what's the relationshipīetween the angle that this is at on the wall,Ĭompared to the center line? Well, we already figured that out. So if this is the first point, let's just say that's one Has to be, what? If I want destructive over here, it's got to be a half wave length, three halves wave length, five halves wavelength. ![]() Of this second wave, this lower middle wave has to travel. Okay, if these are going to interfere destructively Why? Well, we know how to play this game. How about these two? If these two interfere destructively, the argument I'm going to make is if these two interfere destructively, all the rest of them are going to have to interfere destructively. I'm going to also lookĪt the fifth one down. Most, upper most point, has to travel a certainĭistance to get there. So the wave from this top most point, this wave from the top So let's look at this minimum right here. To make this a little bigĮasier to think about. Figure that out? Okay, well this is a- I said there's infinitely How can we get this? How do we analyze it? That's what we're going Up a little bit again, over and over here. These are relatively weak compared to other interference patterns These are going to be not very pronounced. And then jumps back up, and then it comes back up again. The weird thing is, this jumps back up goes to a minimum. Single hole, you would getĪ big bright spot there. Big ol' bright spot, because you're shining a We see a big ol' bright spot, right in the middle. What interference patternĪre we going to see? Well, on the wall over here And since some of them are blocked, we could see an interference pattern over here on the wall, because these can interactĪnd interfere with each other. When we have a Single Slit, we really have infinitely this point's going to start spreading out. Every point's the source of another wave. Slit Interference, because if I get rid of all of that, if we imagine our waveĬoming in here like this. When you've got a hole or a barrier, that's when we actually notice it. We just didn't notice itīecause it always added up. So, it was alwaysĭiffracting, so to speak. When it hits the hole? You just see this thing spreading out. Now that these are blocked, they're not going to get to interfere constructively and destructively You just get this one here spreading out. But when there's a barrier, when there's something in the way, these here can't re-join With the other waves around it and every other point and Every point on here was doing diffraction. ![]() It's because the wave wasĪlready diffracting, so to speak. And this is the key to understanding why diffraction happens. Wave be another wave source, it will just add up toĪnother wave front here. This going to give you? Well, if you add this up, these are going to interfere with each other, constructive, destructive, in a way that just gives you this same wave front right back. Each point is the source of another wave. If, in the forwards direction, this wave spreads out spherically. Or, he realized, with a wave, you can treat every point on this wave as a source of another wave ![]() You can say, "Yeah, that wave front moves from here to there." That's what it does. If you've got a wave front coming in, propagating this way. Remember these wave fronts are like peaks. If you've got a wave coming in, these wave fronts. And I apologize right now toĪll the Dutch people out there, I'm butchering this name. This is a Dutch physicist, scientist, who figured this out. They spread out at a hole is something called Huygen's Principle. ![]() But for a single slit, how are we ever going to get this? Well, I never really told you, why do the waves spread out at a hole? Why does diffraction happen at all? Why, when waves encounter a hole, do they spread out? And the answer to this question is the key to Single Slit Interference. At least with a double slit, you would have two waves spreading out. So how could a Single Slit ever produce multiple waves that could overlap? I mean, when we had a double slit- if I put a barrier in here- and we have a double slit. Single Slit Interference? Interference? Wave Interference is by definition multiple waves overlappingĪt a single point. Now if I were you, I'd already be upset and a little mad.
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