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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

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