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  • almost eight years ago when this channel was fresh and before I had gray hairs in

  • my beard in fact before I had a beard I made a video showing that if you take a

  • grape and cut it almost completely in half and put it in the microwave you can

  • make some plasma

  • but the explanation we had in that video was lacking this was

  • me and my friend Dr. Stephen Bosi a fellow physicist so you should look

  • forward to that on another episode of veritasium to be honest nobody really

  • knew how this effect worked that is until now as of the publication of this

  • video three scientists have published an explanation in the Proceedings of the

  • National Academy of Sciences they studied this effect with high-speed

  • cameras thermal cameras electromagnetic modeling and they came up with an

  • explanation that I think is pretty satisfying and it extends way beyond

  • just microwave great plasma so naturally I called up these scientists yeah

  • actually saw your video before that your videos before joining the grape project

  • oh really that's why I was like already kind of

  • like interested before joining the project the scientists showed you don't

  • even need to use a grape you can get the same effect with hydrogel water beads

  • these are those tiny polymer beads which when you soak in water they increase in

  • volume like 100 times

  • to understand how microwaving a grape gives you plasma we first need to know a

  • few things like a typical household microwave uses a frequency of

  • electromagnetic radiation at about two point four five gigahertz that means

  • that the wavelength of microwaves inside the oven is about twelve centimeters

  • long. now if you've ever studied electromagnetic radiation you might have

  • the intuition that interesting things start to happen when the object or

  • obstacle is around the same size as the wavelength which a grape certainly is not

  • it's not twelve centimeters but the important quantity is how big is the

  • wavelength inside the object inside the grape twelve centimeters in air but the

  • like index of refraction of grape material must be close to like one point

  • three three or something right like at visible wavelengths it is a lot lower

  • but in the microwave regime the index of refraction is a almost 10 so you have a

  • much higher index of refraction so microwaves are traveling you know like

  • ten times slower through the grape than they are through yet air yes and that

  • means that the wavelength is a tenth its size through air so instead of twelve

  • centimeters it's about 1.2 centimeters right about the size of a grape now if

  • you take a single grape and place it inside the microwave don't cut it in

  • half what you find is that the microwaves actually become trapped

  • inside the grape and that's because of its high refractive index and its size

  • because turns out that when you have like a ball where approximately the

  • diameter is roughly the same as the wavelength of the microwaves in the material. It

  • actually turns out that it can be trapped in there so that it interacts its kind...

  • it's kind of it bounces at the borders of the of the ball and it can't get out

  • is it like total internal reflection? yes pretty much wow that's interesting I

  • hadn't thought of it that way so the microwaves become trapped inside

  • the grape and they actually form these resonant modes you can think of them as

  • standing waves just ways in which the electromagnetic fields like to

  • oscillate inside a grape such that the maximum electromagnetic field is

  • actually in the center of the grape when you put in the microwave you see that it

  • heats up not from the outside in like you would expect if it was just

  • absorbing the microwaves but rather from the inside out. if you add a second grape

  • to the microwave you see that the same thing happens the microwaves get trapped

  • in the second grape too- and the amplitude is highest in the very center of the

  • grape and so the heating occurs most of all of there. but if you move these two

  • grapes close together so they're closer than say a wavelength well then you can

  • start to get interactions between the electromagnetic fields in the one grape

  • and in the other and in fact if you get the grapes touching then the greatest

  • electromagnetic field actually occurs at that contact point between the two

  • grapes and so that is where you're gonna get the greatest oscillating

  • electromagnetic fields and that is where those grapes are going to get hottest

  • it's also interesting that you don't need to cut the grapes in half. two

  • grapes placed side-by-side will make this effect work as long as they stay in

  • contact. but that's why in a lot of the videos you see there's a watch glass so

  • the reason the watch glass is there is to keep the grapes together now

  • with the very strong electromagnetic fields at the intersection of the two

  • grapes what you can get is some sparks some breakdown of air that is the

  • electric fields are strong enough that they ionize the air creating those

  • Sparks and that is what leads to the plasma it creates these ions which can

  • then receive more energy from the microwaves. you can see it here pulsing

  • at 120 Hertz that's twice the 60 Hertz frequency of mains power which reflects

  • the fact that the amplitude of the microwaves is pulsing twice every cycle

  • and what is that plasma made of? well the scientists looked at the spectrum of the

  • plasma from a grape and they found that there's a strong potassium emission line

  • and also a sodium emission line so it seems like those ions must be pretty

  • common in the grape and when the plasma gets formed as those ions which are

  • sprayed up into the air and that is what we're seeing

  • who needs drugs?! now the size of the grapes is important which is something I

  • of course suspected because you don't see this happening with too many other

  • squishy fruits but you don't have to get them the exact right size and this is

  • because water absorbs microwaves so the scientists did some modeling of this and

  • if you had a material inside the grape that did not absorb microwaves then what

  • you would see is at certain particular sizes you get extreme amplification of

  • the electromagnetic fields it's basically one size that you'd have to

  • hit to get the amplification but made of an absorbent material like water this

  • broadens out the peaks so you get less amplification of the field but over a

  • broader range of sizes this is why a lot of grapes will work even if the sizes

  • vary. so in terms of like applications of this research what do you think I mean

  • obviously people like me are excited and because this is a fun trick but but

  • beyond that what do you think in terms of like applications of this? the main

  • limit in fabricating microchips right now and going smaller and smaller is the

  • lithography so how to make these very small features. this phenomenon shows

  • that two spheres of the right size and refractive index can focus

  • electromagnetic energy down to a tiny spot in between them in this case down

  • to around a millimeter from radiation with a 12 centimeter wave length if

  • similar focusing could be achieved with light it would provide a significant

  • improvement over current lithography techniques. So if we could somehow harness

  • this to do this lithography to be able to maybe write things very small you

  • could actually use light to to make really small features you could do like

  • every two nanometers marks spot with about 2 nanometers resolution - it would help

  • help continue this like Moore's Law yes it would help continue Moore's Law like

  • putting much more small things in a single chip so I'm glad I was finally able to

  • explain how this effect works did you enjoy it do you feel like you get it any

  • questions put them in the comments below also I

  • really like this video that I made this is my previous video and so if you

  • haven't seen it go check it out I'll also put a link to the original

  • grape video which is maybe not that good cuz it was eight years ago but anyway

  • thanks for watching

almost eight years ago when this channel was fresh and before I had gray hairs in

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ブドウを電子レンジでチンしてプラズマにする方法 (How Microwaving Grapes Makes Plasma)

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    林宜悉 に公開 2021 年 01 月 14 日
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