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