 So remember you get interference when waves add up. If waves add up in phase like this Then you get a big wave that's constructive interference and if they add up out of phase like this Then you get nothing. That's destructive interference Now if you have two sources of waves for example like the two red bubbles in our water tank and they're both giving out waves of the same frequency then those waves are going to meet and Depending on where they meet they're going to be either in phase or out of phase Imagine the sources they're going up and down at the same time. So they start off with the waves in phase However, of course, they're going to go different distances and if one travels one distance The other one travels different distance Then they might go either in phase or out of phase So the distance the extra distance that one has to go supposing they they're in phase at one point This is how far it has to go to go out of phase has to go from there Over to here now. How far is that? We've taken this point here from over this point to over that point. And so what is that distance? Well, this distance here from peak to peak is a wavelength and so this distance here must be half that So this is half a wavelength So whether or not you get constructive or destructive interference depends on how much further one wave travels compared to the other So if you have a wave traveling from one source to a point and then you look at how far the wave has to go from the other Source to get to the same point this point is where they're going to add up And so we're either going to get constructive destructive interference or something in between and that depends on what's called the path difference The difference in how far the two waves have to go to get there So this distance here if that's an odd number of half wavelengths Then they're going to be out of phase and you're going to get destructive interference If that's an even number of halfway of things and they're going to be back in phase and then going to have constructive interference So if we look around this area, we'll find all sorts of different points And if we draw the distance that the wave has to go from each source to get to each different point Then each of these points we're going to get different path length differences So we get different constructive and destructive interference and then what we get is a big interference pattern So for waterways where you have destructive interference You'll have points in that interference pattern where you have waves coming in but there's no actual move into the water If you have sound then you'll have sound coming from two speakers and it'll exactly cancel and so you have a quiet area You could only actually get this effect when the thing that's traveling is actually a wave Destructive interference requires it to be a wave if I was throwing actual particles together Then you'd always just get lots of particles where they meet doesn't matter whether they're in or out of phase That doesn't mean anything