How To See Air Currents

If there were a portal through which you could see all of the invisible air currents, temperature gradients, and differences in pressure and composition of air, then this is what it would look like to strike a match. This is helium being squeezed out of a balloon. You could see the warm air rising off your hands, the invisible vapors of isopropyl alcohol, and a plume of ejected material when you sneeze.

This is the setup for how I made those shots over here. I have a 40 centimeter diameter concave parabolic mirror. Normally, this would be used to make a telescope, but if it forms just a tiny piece of a big spherical shell, right? A giant spherical mirror that we're inside. Then the center of that mirror would be right here, and that's where I've placed this LED. It's three millimeters, but I try to make the light source even smaller by painting it with some black nail polish. So, we try to approximate a point source of light there.

Now that light spreads out in all directions, and it bounces off the mirror and reflects back almost to exactly the point where the light is. You can see that the light converges to a point right there. Now I've offset the light just a little bit so that this light will pass straight through and into the lens of my camera. So right here, I position this razor blade, so it cuts off about half the light passing through.

This setup allows you to see tiny variations in what is in front of the mirror. For example, when you have hot gases coming off of a candle, you can see that because as the reflected light from the LED passes through this column of hot air, it changes directions ever so slightly. That is, it refracts, and the reason for that is because the refractive index of hot air is different from the refractive index of the cooler air around it.

Now, the refractive index is a measure of how fast light travels through a medium relative to its speed in a vacuum. So, for air, the value is pretty close to 1. But hotter air actually has a lower index of refraction. In this case, the difference in refractive index is incredibly tiny, and so we don't notice the deflection of the light. But with this setup, it actually makes a difference because some of the light that would have passed over this razor blade instead gets deflected down and gets blocked, and that forms a darker spot on the image.

Similarly, some light that would have hit the razor blade is instead deflected over it, creating a brighter spot on the image, and that is how this works. So you can see the heat rising off your head, and you can see your breath, and you can see the cold air poured out of a cup of ice. But temperature is not the only thing that affects the refractive index; different materials, different substances, have different refractive indices.

For example, the butane in a lighter, obviously, we can't see that it's coming out right now, but the camera can. Even before the lighter is lit, light also refracts when it passes through a bubble, and the amount depends on the thickness of that bubble film. This technique is known as Schlieren, based on the German word "flir," which means streak, and it was first observed in 1665 by Robert Hooke, who was using two candles and some lenses.

Then, in the 19th century, they used this method to try to find defects in the glass used to make lenses, and more recently, people have used Schlieren to study aerodynamics and fluid flow because it allows you to see those pressure differences and temperature gradients. So you can look at shockwaves and differences in the composition of gases.

So when you watch the lighting of this match, you're seeing heat generated from friction igniting phosphorus, which in turn generates more heat and begins the reaction between sulfur and potassium chlorate, which releases sulfur dioxide, which you can also see. And you can see my breath as I blow out the flame. I am so excited that I got this setup to work.

So if you can think of anything that would look really cool in Schlieren, then let me know in the comments, and I will try to make it happen. And if you're new to this channel, we'll click here to subscribe! I've got some awesome videos coming up very soon.