BTEC Applied Science Unit 1 Physics and this video is about interference of waves. So there's Derek the duck and he's sat on the water and he's having a very peaceful day having a bit of a snooze but now here is a point source so this is producing water waves now this point source might be somebody dipping their finger in the water yes or throwing stones in the water or something dipping in the water but it's producing circular wave fronts so we see the waves traveling out from the point source uh-oh they're heading towards Derek he's scared he doesn't know what's going to happen so here comes the wave what's going to happen to Derek what do you think's going to happen well Derek will oscillate yeah he will oscillate up and down yeah you'll go up and down and his amplitude will be the same as the amplitude of the wave if you've been watching my videos in the right order you'll know what amplitude means this is fun he's having a nice day now What's going to happen if there are two sources of waves, two point sources, and these sources are coherent? Now, what does that mean?
You need to learn this. If two sources are coherent, then they have the same frequency, they have similar amplitudes, doesn't have to be exactly the same amplitude, but similar amplitudes, and they are in phase. Okay, so they're producing waves together.
They're going up and down together producing waves. So the waves that are leaving the source are in phase as well. And I've talked about phase in another video.
So these are two coherent sources and be able to say what that means. So what if the waves arrive at Derek from two different sources what's going to happen to Derek and the answer is there are different possibilities now will Derek oscillate with the same amplitude as before in other words if there was just one point source Derek was bouncing up and down with this amplitude a if there are two sources will it be exactly the same or will this amplitude be bigger you Will the waves add together and then the duck oscillates with twice the amplitude? So the waves are adding together.
Now that would be constructive interference. If the waves add together and the resultant amplitude is bigger, that is constructive interference. Or will the waves cancel each other out? So that Derek doesn't actually do anything or he just does very, very little. And if the waves cancel each other out, that is destructive interference.
So constructive interference is when the waves add together and the amplitude gets bigger. Destructive interference is when the waves add together and cancel each other out. Now, the type of interference, whether it's constructive or destructive, depends on where the duck is. It depends on his position. OK, in some places it's going to be constructive and Derek's going to be bouncing up and down like crazy.
In some places it's going to be destructive and Derek will hardly move at all. Looking on this diagram, I know that here. In fact, anywhere along that line, it's going to be constructive. And then perhaps over here, it'll be destructive. So this derrick over here won't do very much, or this derrick won't do very much either.
And then there'll be other places where it's constructive and destructive. Now, why? Well, the two waves started off in phase.
Remember that the two sources were coherent, so the waves started off in phase, but It depends if they arrive in phase or not. Now, if they arrive at a particular point in phase, so the waves are going up and down together, look at the diagram, then we get constructive interference, and the amplitude is bigger. If they arrive in antiphase, so one's going up while the other's going down, so they're kind of moving opposite to each other, then we get destructive interference and they cancel each other out. So it depends on whether they arrive in phase or whether they arrive in antiphase. And whether that happens or not at a particular point depends on something called the path difference.
Now the path difference is basically if you look at how far... the waves have traveled so if this is one source here let's say that's s1 and this is another source here and let's say that's s2 so there's my two sources s1 and s2 and if we look at a point let's say over here now this wave has traveled further than that wave hasn't it to get to that And the difference is this. That is the path difference.
So it is the difference in how far the waves have travelled. Now, if the path difference at a point equals zero, for example, at this point here. Now, at that point, the waves have travelled the same distance, haven't they? So the path difference is zero. Or if the path difference is a multiple of wavelengths.
So if the path difference equals lambda or two lambda or three lambda, if it's a multiple of wavelengths, then the waves will arrive in phase and you will get constructive interference. What will determine whether the waves arrive in phase or not? It is the path difference. If the path difference is zero, then they will arrive in phase, because they've travelled the same distance.
If the path difference is a multiple of wavelengths, then they will arrive in phase, and you will get constructive interference. Now, get my rubber out, all that. get rid of all that it's in the way sorry if the pass difference is half a wavelength or it's like a wavelength plus half a wavelength or two wavelengths plus half a wavelength if the pass difference is half a wavelength then the waves will arrive in antiphase and you will get destructive interference This is a fantastic animation and you should play with it.
It's a FET simulation called wave interference and just google it and this represents two loudspeakers and you've got two lots of waves coming out they are coherent sources we know what that means and when the path difference is zero which is anywhere along that line the path difference is zero because the waves have traveled the same distance so the waves will arrive in phase and you will get constructive interference in the case of loud speakers if you were stood there it would be really loud yes along this line here the path difference is a wavelength so again the waves arrive in phase and you will get constructive interference and along this line here the path difference is a wavelength there's another line there there's the path difference is two wavelengths and down there it's two wavelengths and this is constructive interference the waves start off in phase and they arrive in phase now looking at this diagram there the path difference was zero in the middle Along this line here, can you see there's not a lot happening? And that's because the path difference is half a wavelength. Okay, and because the path difference is half a wavelength the waves arrive in Antiphase and they cancel each other out we get destructive interference and here the path difference is half a wavelength Here the path difference is a wavelength plus half a wavelength and down here It's a wavelength plus half a wavelength.
So if the waves arrive in antiphase then we get destructive interference. If you get a question on this topic, and they do come up quite often, basically make sure that you use these key words. The person marking the question will be looking for these key words. And the key words are, when waves from coherent sources.
You might be asked to explain what coherent means. When waves from coherent sources interfere, if the waves arrive in phase, then we get constructive interference. This happens when the path difference is zero or a multiple of wavelengths. That's constructive interference. You do destructive interference yourself.
When does that happen?