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  • A few months ago, I made a video about Schlieren imaging.

  • Now that's a technique used to visualise tiny differences in air either temperature, pressure, composition,

  • so you can see things like the heat that comes off when you light a match.

  • Now in that video I asked you for some ideas for what you'd like to see in this Schlieren setup,

  • and I will be showing you some of those in this video.

  • But I also saw that there was a lot of confusion about how exactly the setup works

  • and so I want to clear up that confusion

  • before we get going. So one thing is this mirror.

  • I said is a parabolic mirror, but then I said you can think of it as a part of a sphere

  • and so people rightly called me out and said well if it's parabolic it can't be part of a sphere. While that is true,

  • it's such a small part of

  • a sphere or parabola

  • that the two shapes are pretty similar on this scale,

  • so you could say it approximates a sphere

  • and you would not be as far off as say in physics when we say this cow

  • Approximates a sphere. Now a parabolic concave mirror is a pretty good approximation for a spherical concave mirror. Now the second major

  • question is how do you get enough light to make this work. I mean some of those shots

  • I was showing you were shot at 2,000 frames per second and using only one tiny LED as the light source.

  • How could that even work? Well, I had similar kind of concerns when I was going to set this up

  • and so I sought out the most powerful flashlights that I could find.

  • I figured this would be my light source so that light bounces off the mirror. Now the focal length of my mirror is

  • 1.8

  • meters which means that the light converges at the center of curvature,

  • which is two times the focal length, 3.6 meters back

  • and you can see that here the reflected light that comes off the mirror is

  • converging on this card that I'm holding up. You can see as we get closer and closer... right there look at how sharply...

  • We have an image of the flashlight. It passes and

  • forms a spot back here.

  • This is essentially an image of the mirror and look how bright it is. We have daylight outside. Okay,

  • that's very bright, but you can still see this spot, which comes completely from the flashlight. Now

  • let me try to set up something in front of the mirror that will produce a bit of a schlieren pattern and let's check

  • how that looks. You can actually see the schlieren effect, and you might notice this is not a

  • fantastic schlieren image, and that's because, of course,

  • this is not a point source of light

  • so my first thought was cover it with tin foil and make a hole on the front to reduce the size of

  • the light source. Now that has decreased the brightness of this image, but it's still pretty bright and

  • you can actually see a really nice

  • flaring effect that's taking place

  • already right here, so if you imagine sending all of that light down the lens of a camera,

  • it's actually a pretty bright image, not a dim one, but if this works and produces the schlieren effect

  • we're looking for then why do you need the razor blade? Well,

  • here's the thing. The differences in refractive index that we're talking about are very very tiny, so

  • the way the light is deflected is just by the tiniest of degrees,

  • So that light actually will end up right here

  • in this focal point with all the rest of the light because all of the light from the mirror is getting sent back to this spot.

  • But some of it the light that's been deflected will be just ever so slightly off

  • and if you use this razor blade to knock off about half of that bright spot,

  • you will be cutting off more of that light, which has been deflected and by cutting it off,

  • you will increase the contrast in the image.

  • Now, as some of you pointed out, an alternative to using the razor blade to cut the light is

  • using colored filters, so here I got two different colors of cellophane

  • and if I position that so that the

  • focal point is right in the middle of the divide then some of the light will pass through this

  • magenta side and some will go through the cyan side. Have a look what happens when I put this

  • transparent helium balloon in front of the mirror. Now the helium just deflects the light a tiny amount,

  • but it's enough so that instead of going through the magenta side it goes through the cyan side

  • and that's why the balloon looks a different color, but what happens if I pop it?

  • Do you see that after the balloon pops, the helium kind of stays together and rises in the shape of a balloon?

  • Another thing you suggested was lighting a barbecue lighter with a match, so I'm just gonna release some gas

  • I'm gonna let the flame travel up the gas

  • and the schlieren you can see the flame traveling out and through that gas up to light the lighter.

  • Yeah. I'm gonna try blowing a bubble with rainbow schlieren.

  • Another crazy idea of yours was to light a ping-pong ball on fire because

  • ping-pong balls, the good ones anyway, that they use in actual tournaments are quite flammable.

  • So I'm gonna see how that looks. Whoa! It's caught fire!

  • Look at it go! Oh that is awesome!

  • Now a lot of your recommendations involve sound, for example, trying to see a clap. I

  • tried this again and again

  • and I tried looking for a shock wave, but it was pretty hard to see even at 2,000 frames per second

  • and that's because with the speed of sound being over 300 meters per second, even at that speed you would only catch

  • maybe one or two

  • frames. That would include the shock wave in them

  • and I found it almost impossible to see. So looking at all these claps,

  • you can see the air getting pushed out from between my fingers,

  • but that's not traveling at the speed of sound, so it's not the actual sound of the clap.

  • So I'm gonna have to call in some backup if we're going to do some schlieren

  • with sound. So thank you for all of your suggestions. If you have any others,

  • please leave them in the comments below, though it may take me a little while to get back to you because next week

  • I'm flying to Australia and on the plane

  • I will be listening to the sponsor of this episode of Veritasium,

  • which is, of course, Audible. Audible has an unbeatable selection of audio books read by

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  • I think it's really the best way to get books into your life

  • and if you're viewing this video, Audible will offer a free 30-day trial to you. By going to audible.com/veritasium,

  • you can download an audio book of your choosing for free, so that's a pretty cool

  • deal and the book that I'm listening to at the moment is

  • Algorithms To Live By. This is what I'll be listening to on the plane.

  • It's about the intersection of computer science and human decision-making ,so if you're trying to optimize your life

  • how can the lessons of computer science apply to making your life better?

  • You know I'm fascinated by

  • optimizing my life and also by computer science, so this is a really cool book.

  • I'm enjoying it so far, and I can't wait for that plane ride.

  • I find that having an audio book really

  • changes the idea of a commute and makes it go from a real drag to being something that I kind of look forward to because,

  • I wanna get back to my book.

  • So you should try it out. You can download any book you like including that one for a free 30-day trial by going to audible.com/veritasium.

  • One last thing is that Audible is

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  • I wanna thank Audible for supporting me and I want to thank you for watching.

A few months ago, I made a video about Schlieren imaging.

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カラーでシュリーレンイメージング! (Schlieren Imaging in Color!)

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    林宜悉 に公開 2021 年 01 月 14 日
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