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 form, 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 millimeter

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

Well, 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 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

They 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 and 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 in 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 set up 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