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.