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  • 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

If there were a portal through which you could see all of the invisible


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B2 中上級

見えないものを見る。スローモーション・シュリーレンイメージング (Seeing the Invisible: SLOW MOTION Schlieren Imaging)

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    林宜悉 に公開 2021 年 01 月 14 日