A Supporter of mine on patreon - Zack Murphy - noticed something strange while handling helium balloons. If he put one between himself and a distant sound the sound got quieter and importantly it's not Something that happens with normal air-filled balloons. So what's going on? My theory is that it's a form of lensing So you can use a lens to focus light light travels as a wave and in principle You should be able to focus any kind of wave so long as you can build the equivalent of an optical lens in our case an acoustic lens or sound lens. And look these helium-filled Mylar balloons. They kind of looked like lenses Specifically convex lenses as opposed to concave lenses But here's the weird thing: In optics a convex lens focuses light so if you're looking at a light through a lens It should appear brighter so long as you're somewhat near the focal point Except that's not what's happening with helium. With helium the sound isn't getting louder, It's getting quieter. Which is equivalent to a light getting dimmer when you put a lens in front of it So there's something more going on. The reason a convex lens focuses light is because glass is more Optically dense than air or it has a high refractive index - to use more formal terminology when light passes from a low refractive index medium to a high refractive index medium like from air to glass the light bends towards this Perpendicular line sometimes called the normal in optics There's more than one way to explain why light bends in this way But the intuitive one that I like involves looking at the wavefront It's important to know that light travels more slowly in a medium with a high refractive index So look as the light goes from air to glass it slows down And so the peaks and the troughs of the wave need to bunch up a little bit. In other words the wavelength shortens if the light comes in at an angle Then this slowing down of light actually changes the angle of the wavefront and if we assume that light always travels Perpendicular to the wavefront then the direction of travel must change as well The opposite is true when you go from a high refractive index to a low refractive index medium in other words, the light bends away from Perpendicular away from normal. So hopefully you can see that the geometry of a convex lens will cause Parallel rays of light coming in to bend inwards to a focal point If you wanted to defocus the light then you would use a concave lens and hopefully you can see From the geometry here that that's what you get. So why does a convex sound lens appear to be behaving like a concave optical lens Why is it spreading out the sound and Reducing the volume at any point in space. Well, it's because helium is less Acoustically dense than air which is the opposite of what we have with glass and air where glass is more optically dense than air Helium is less Acoustically dense than air so we need to switch the geometry around to get the same effect so hopefully you can see that here as the rays of sound enter the Convex geometry of the balloon the rays bend away from normal and then they bend Towards normal on the way out Let's test this hypothesis by trying to build a focusing sound lens we can do that by building a concave helium lens or We could build a convex lens that's filled with a gas denser than air The easiest to get hold of is carbon dioxide - so that's what I'm going to use And the easiest way to supply carbon cioxide is with a fire extinguisher A quick detour about lens geometry which is important for the experiment this surface here is a Section of a sphere so you can imagine a big sphere like this and we've just got a part of it here and that bottom surface is also a section of a sphere And the reason the geometry of a lense tends to be spherical is because it's easy to make. Imagine you've got a Spherical indentation and you've got a rough bit of glass and you're grinding it inside that indentation It doesn't matter how you orient the lens because it's a spherical indentation. That wouldn't be true for other geometries but it turns out that Spherical geometry isn't the perfect geometry to focus light to a point. what you actually want is one surface to be elliptical and the other to be hyperbolic But for a thin lense like this It's a pretty good approximation So spherical geometry is fine But the lens that I've got is a full sphere and you're going to end up with something called spherical aberration where you don't get perfect focus, so I also wanted to see if I could create a more traditional lense shape, so I did that with a mylar balloon, but where I reinforced the rim with copper pipe That's so that it wouldn't become rounded around the edge if you're wondering how I got the copper pipe in the balloon I do it like this To test the lens I need a source of sound I used my phone for that and I've got some software that generates white noise I skewed the white noise towards higher frequencies That's because for the lens to work the wavelength of the sound needs to be substantially smaller Than the geometry of the lens that's true for optical lenses as well by the way And so anything below about a thousand Hertz Probably isn't gonna work a thousand Hertz is a wavelength of about 35 centimeters Ironically, it sounds just like a balloon deflating. I did some rough calculations to figure out the focal length of the lens just to make sure that I could do the experiment in my garden Like I wasn't gonna have to go tens of meters away from the lens to find the focus and that bit was fine But to pinpoint the exact focal point I needed to do some trial and error With the microphone mounted on the lens You can hear the moment that I hit the sweet spot the focal point of sound with my camera And here it is with the more traditional lens-shaped balloon I actually think the spherical balloon is working better perhaps because it's larger so it's gathering more sound I'm going to show you the yellow balloon experiment again this time from the perspective of the handheld camera and at the same time I'm going to show you a frequency analyzer I'm going to freeze the frequency analyzer at the low point of amplitude and at the high point of amplitude when we hit the focal point And there you go you can clearly see the difference in amplitude it really does seem to have a lensing effect For completeness here's the mylar balloon filled with helium and you really can hear the amplitude go down as the sound is defocused So, there you go Thanks Zack for the idea and thank you to all my patrons on patreon for your support and your ideas Here's a question for you. Do you use Chrome browser? statistically speaking You probably do and I used to but I don't anymore you might think that switching browsers is a massive pain in the arse But I wanna explain why it doesn't have to be and it's really interesting actually So Chrome is not an open-source project But it's based on an open-source core called chromium and other people can build browsers based on this open source chromium, for example the sponsor of this video brave and the great thing about a chromium based browser is Chrome extensions work in chromium based browsers So if you've got your Chrome browser set up just how you like it with all your extensions and everything You can transfer your extensions your settings and your bookmarks Over to a browser like brave in a minute, but why would you switch well? chrome isn't a privacy based browser a Part of Google's business model is to know as much about you as possible Brave is a privacy focused browser. It blocks all that kind of tracking it blocks fingerprinting It blocks third-party ads because these third-party ad networks tend to be the ones that are interested in Getting to know you and following you around the internet. The upshot is that brave is faster, and it uses less data There's an Android version of the app as well So you can have everything in sync it even saves battery on a mobile phone go to brave dot com forward slash Steve mould the link Is in the description as well normally at this point in a sponsor message I'd say hey get 30% off the link in the description, but Brave browser is completely free and open-source, so I can't say that All I can say is yeah, please use that link because that helps me out It lets them know that I sent you download brave today I hope you enjoyed this video if you did, don't forget to subscribe and I'll see you next time
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