B2 中高級 7 分類 收藏
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 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


Seeing the Invisible: SLOW MOTION Schlieren Imaging

7 分類 收藏
林宜悉 發佈於 2020 年 3 月 29 日
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