How Microwaving Grapes Makes Plasma

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

Channel: Veritasium

Views: 7,900,853

Rating: 4.8973365 out of 5

Keywords: veritasium, science, physics, microwave, grape, plasma, spark, microwaving, explanation, explained, how, index of refraction, resonance, standing waves, electromagnetic radiation, wavelength, experiment, grapes, glowing, glow, light, absorption, hydrogel, sparks, size

Id: wCrtk-pyP0I

Length: 8min 15sec (495 seconds)

Published: Mon Feb 18 2019

Reddit Comments

[u/BrokenEye3]

Whelp, I'm off to do some science.

[u/49orth]

Near the end of the video (worth watching the entirety) they talked about the possibility of using the hypothesized phenomena on lithography etching of computer chips down to 2 nm.

[u/supremedalek925]

Oh, so that’s why we don’t eat hot grapes.

[u/Zorak6]

Now I want to see a sci-fi series where they have to rearm their plasma pistols with grapes.

[u/Trainheart3]

Would it be possible to accomplish this with a banana somehow

[u/Phrankespo]

Video...NOW! I'm out of grapes!

EDIT: I overcame laziness and googled. here you go. https://www.youtube.com/watch?v=PGX289yjew8

[u/jesuzombieapocalypse]

Does it damage the microwave to do this under a glass? Because I really want to do this.

[u/dontregredditt]

green or red grapes?

[u/riggidyrektson]

r/MBMBAM