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Schlieren optics allow us to see the movement of air

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So, what is "Schlieren Optics" and how is it letting us see this cool stuff?

593 points · 2 months ago

Instead of seeing differences in intensity of the light like with a normal camera system, we are seeing differences in the density of the air, which lead to changes in refractive index.


14 points · 2 months ago


18 points · 2 months ago


5 points · 2 months ago

I think that’s what Martha Stewart practices.

What could we do with this?

See ghosts

Oh shit


Found the ghost.

Not without Schlieren Optics you didn't

And farts!

Which is exactly why they don't use these on ghost hunter shows.

Rekt. Ghost Hunter shows.

And farts

That’s great

11 points · 2 months ago

See farts.



Finally find out if he who smelt it truly dealt it.

Are you asking for practical applications?


In the interest of giving you actual information and not being wrong, I lifted this from Wikipedia.

A typical application in gas dynamics is the study of shock waves in ballistics and supersonic or hypersonic vehicles. Flows caused by heating, physical absorption[6] or chemical reactions can be visualised. Thus schlieren photography can be used in many engineering problems such as heat transfer, leak detection, study of boundary layer detachment, and characterization of optics.


Schlieren photography

Schlieren photography (from German; singular "Schliere", meaning "streak") is a visual process that is used to photograph the flow of fluids of varying density. Invented by the German physicist August Toepler in 1864 to study supersonic motion, it is widely used in aeronautical engineering to photograph the flow of air around objects.

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Here is an interesting application of Schlieren optics that you won't hear about every day, it can be implemented in microscopes to visualize transparent organisms and structures.

At the microscopic level it can be very difficult to see organisms. Not only because of their smallness, but also because many microscopic organisms are nearly transparent. This makes differentiating them from their background difficult. Dark field, phase contrast, and interferometry are normally used in such situations. But each has their drawbacks so schlieren optics become attractive for specific situations.

So you'll see the denser microorganisms next to the relatively still air? Or do they make waves?

With the right setup you could see both, although the aim is usually to see the internal structure of an organism this technique could allow you to see, for example, the flow of fluids.

3 points · 2 months ago

They're commonly used in wind tunnels, especially high speed ones. That's where I've learned about them and used them. Cool form of flow visualization but can be really hard to setup.

Find out where that draft is coming from

Design cars

3 points · 2 months ago

A post-doc I worked with who was an Aerospace Engineer at NASA for a bit used it to visualize airflow patterns for the space shuttle.

Track xenomorphs.

motion tracker from first alien movie?

"micro changes in air density"

I understand all of those words individually

Not sure if you’re just kidding but in case other people are curious, but also lazy, the refractive index is how much a material will bend light. As the air changes density, so does the amount light passing through it is bent.

Coatings on glasses affect the refractive index to make your lenses less reflective. The refractive index of a prism splits white light into a rainbow.

3 points · 2 months ago

I almost felt proud of myself for understanding... refractive index wat

134 points · 2 months ago

Veritasium has an incredible video about it and fully, clearly explaining how it works on his channel. (No link cuz I’m a filthy scrub on mobile)

Edit: fiiiiiiine here.

Thanks for posting that! Cool video, He explains it quite well!

8 points · 2 months ago · edited 2 months ago

Daaaaaamn, that was insane! How much does a mirror like that cost?

Edit: Would this one do the trick? How do I know how far away the center would be based on the data given?

Website says focal length is 900mm (~3 ft). Im 90% sure that means the center point. Also note that the mirror is only 4.5 inches, its pretty tiny.

3 points · 2 months ago

In this sequel video (at 1:36) he says that the one he is using has a focal length of 1.8 meters, and the center of curvature, where he places the camera, is 2 times that (see this for background). He still definitely uses a larger mirror, but the scale of the difference isn't as big as I expected.

TIL. Thanks. Hence the 90%, i knew i could be wrong.

No, focal point is where he put the camera and light source. 3' is probably not going to work.

I don't see why not. The differences would probably be not as obvious as the light wouldn't have as far to alter course but should still be visible.

-17 points · 2 months ago(0 children)

That's a great video. Could you use a laser pointer instead?

A laser pointer is a pretty good point source, but it also has a very small spot unless you put more optics in front. An LED is a better choice.

Here's a video demonstration. A point source of light is reflected from a concave mirror and focused onto the edge of a razor blade, which is mounted in front of the camera. Light refracted near the mirror and intercepted by the blade gives the illusion of a shadow. Seen here are the heated gases from a candle flame and a hair dryer, helium gas, and sulfur hexafluoride gas.

Why a razor blade? Is it just that it happens to be the right shape for this or is there something specific about it being a razor blade that makes this work? Still not sure what’s going on here as that video didn’t explain anything that was going on in it.

Yes the razor blade is used because it has a very sharp edge while being cheap.

I didn't watch the video so i don't know what they tell you or don't but I'm actually writing parts of my bachelor thesis about it so i can give it a try explaining.

The light is made parallel with the mirror which is parabolic. It's not perfectly parallel (because the light source isn't a perfect point source) which is why you can imagine the light beams not as a perfect mathematical line infinitely thin but rather a beam with 1mm in width, this will be important in a second. After being made almost parallel the light travels through the test area, in this case the area around the candle. If there is a gradient in density, or simply put density changes, the light is refracted meaning it won't travel in the same direction anymore but slightly bent away from its initial direction. Then there is a lens or another parabolic mirror, either will focus the light so all the beams meet in a small focus point. This is where the razor blade comes into play. Remember how the beams weren't perfectly thin but had a small width? When focused this means they all pretty much go through one point into a small circle rather which is where you hold the blade. You hold the edge of the razor blade in there so it cuts off say roughly half of the light coming through. Now remember some of the light got refracted by stuff going on in the test area, these light beams won't hit the focus point perfectly because they changed their direction slightly. Some of them are going to get cut off more by the razor blade and some less, depending on the direction they were bent in, towards or away from the edge of the blade. If you cut off more from a light beam that means that less light will keep traveling on after the razor blade, in this case after being straightened probably directly into a camera. So in a way we made the pixel in one point less bright because we cut off more light and in another point brighter because we cut off less. With some points being brighter lit than others you get an image like in the video with the brightness corresponding to the gradient strength, ie. how drastic the change in density is.

Hope this was understandable, it's a little tricky without any sketches. And i cut the part with the razor blade a little bit short but it should be enough for the principle.

AFAIK just because it’s a commonly-available object with a very straight edge.

Wow, how does someone figure out how to do this? My best guess is smoking some good weed then going to the lab.


Not even once.

7 points · 2 months ago

Some of that good Antiguan ganja.

A simpler version of this effect is shadowgraph and it occurs in nature all the time (shadows at the bottom of a pool for example). Adding the razor blade makes it a schlieren image. So i would suppose they were just experimenting with shadowgraph anyway when they had an idea.

Or just thought experiments. Einstein did his best work that way. The weed bit isn't necessary.

I have so many farts I want to try this on

Just avoid trying it with the sharts

Human eye allows us to see the movement of sharts

Don’t get too close or you may get pink eye

Pink is overrated.

-13 points · 2 months ago(0 children)

For science

Came here for fart jokes Not the same technique, but it does the trick

fake news. why is the fart colder than the air?

Ice cream farts

Comment deleted2 months ago(10 children)

Surprise! Air and water are both described by the same mathematical models—mainly the Navier-Stokes equation. For simplification water is usually treated as incompressible, which reduces the complexity of the equations, but both gases and liquids flow in exactly the same way. Neat, right?

So our atmosphere is like an ocean of gasses on the surface of the planet?

We quite literally live at the bottom of an ocean of air.



Things are a tad different cause gas is really really compressible, lighter (thus easier to move), and changes temperature more easily, but yeah essentially!

3 points · 2 months ago

Untill you move more than .3ish Mach, then you start to take compressibility of air into account!

What if you add some racing stripes?

That's only +1 Intimidation.

2 points · 2 months ago

Neato torpedo

Liquids and gases are both fluids.

That is amazing. Thanks for sharing this

I would like to see it with a cold gas, they only show it with air that is heated.

Schlieren imaging allows you to see differences in air density due to a change in the air’s refractive index. So you can only see changes in the air density, not absolute temperatures. Cold air would not look any different than warm air; what looks neat is when you can see warmer air next to cooler air, so that’s why they’re showing that.

9 points · 2 months ago · edited 2 months ago

I got ya. So if you had 2 air sources at very similar temp gasses it would not be such a dramatic effect. Or if you used helium and air at the same temp it would look similar to what they did. Should have looked it up before I posted

11 points · 2 months ago · edited 2 months ago

Should have looked it up before I posted

Nonsense. You basically came up with another way to test what you're seeing to learn more. That's great as that's what leads to research.

Then someone who happened to already have the answer helped fill in what you would have learned. That's great, it saved you time.

If anything you should be posting more stuff like that.

this is such a kind hearted response. it seems like a lot of people on reddit ridicule people who are too lazy to look up something and just ask for the answers here. encouraging them to be curious and bask questions is a much better approach and i think more people on reddit should be doing that. have a nice day!

Not to mention it might have been difficult to google without understanding exactly how this works anyways. Something tells me “Schlieren cold air or hot air” and the like would not give exactly the answer to the question. Instead you’d be directed to lots of white papers and technical descriptions without a simple answer to “what does cold air look like?”

I feel like I should have offered an even better answer. Your response about helium and air is correct, because they have different densities you should see where they separate, even if they are the same temperature. For example if the helium is allowed to settle at the top of a box and the air below it, you should see a clean line where thy stratify.

But in these warmer air vs. cooler air examples, specifically the candle, you can see natural convection where the hot air rises above the candle and cooler air comes down to replace it. If you put much colder air in front of these optics, you would see it clearly moving downwards to settle since colder air sinks and hotter air rises.

I get the watch part because this is cool, but I'm not learning anything

6 points · 2 months ago

Someone please explain how this works.

It sees air

This guy explains.

It can see air

I look at you and you got those schlieren eyes!

“I don’t want to be human. I want to see gamma rays, I want to hear X-rays, and I want to smell dark matter. Do you see the absurdity of what I am? I can’t even express these things properly, because I have to—I have to conceptualize complex ideas in this stupid, limiting spoken language, but I know I want to reach out with something other than these prehensile paws, and feel the solar wind of a supernova flowing over me. I’m a machine, and I can know much more." -Cavil

I cry everytime

No reason to get excited

1 point · 2 months ago

No shit. And lightbulbs produce light, but that doesn’t explain how it works smartass.

It sees the air..when it moves it sees it

Where can I buy one of these

This is actually terrifying af. We are literally “submerged” in air.

I breath this stuff?!

Ohh you vape too?

This some good shite

I want to see someone breathing in front of this

That small candle moving so much air. It would be cool if some kind of glasses or camera could do this

That is actually really cool!

The same basic method is used to test the surfaces of telescope mirrors and lenses. An uneven mirror will show up with dark and light patches on it. Foucault test

If they would have had one of these in the movie "Twister" they wouldn't have needed "Dorothy" and that lawyer wouldn't have gotten eaten on the toilet by the t-rex.

Here's a sneak peek of /r/NotKenM using the top posts of the year!

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How does it work? Is the mirror coated in something? Is this new or old science?

This is “old-ish” science; the technique has been around for 150 years. It’s actually surprisingly low tech—it just requires a razor blade and some excellent mirrors. Other redditors have given great descriptions of how it works already, so you can check those explanations out above, but the neatest thing I have seen from Schlieren optics is a shock wave on an aircraft traveling faster than sound:

Yes I can finally prove I'm the master of ventilation

Way to breath, no-breath.

I need more of these

Micro-changes in air density, my ass

Lets see a great fart

1 point · 2 months ago · edited 2 months ago

Videos in this thread: Watch Playlist ▶

Seeing the Invisible: Schlieren Imaging in SLOW MOTION+128 - Veritasium has an incredible video about it and fully, clearly explaining how it works on his channel. (No link cuz I’m a filthy scrub on mobile) Edit: fiiiiiiine here.
Schlieren Optics+112 - Here's a video demonstration. A point source of light is reflected from a concave mirror and focused onto the edge of a razor blade, which is mounted in front of the camera. Light refracted near the mirror and intercepted by the blade gives the illus...
Thermal Camera Fart+4 - Not the same technique, but it does the trick
Schlieren Imaging in Color!+2 - In this sequel video (at 1:36) he says that the one he is using has a focal length of 1.8 meters, and the center of curvature, where he places the camera, is 2 times that (see this for background). He still definitely uses a larger mirror, but the sc...
See The Invisible+1 - For flatulence: Starts at 1:22
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Is that Trinity!?

Vsauce Michael here


Can’t believe there are no kinks in that hairdryer cable

Wow, that is amazing!


Farts on it

This is fucking amazing

That’s pretty much how I imagined air moves

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For flatulence:

Starts at 1:22

Can it register ghosts?

Good question—assuming a ghost had a different density than the surrounding air, this would hypothetically show it.

So does smoke

His chick looks serious with her hair dryer

0 points · 2 months ago

There needs to be oven doors with this on it


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