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You’ve probably seen the videos on YouTube turning grapes into fireballs in the microwave. Well, there’s a pretty cool scientific explanation for why a grape is perfect for making plasma.

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Sources:

https://wonders.physics.wisc.edu/measure-the-speed-of-light/
https://van.physics.illinois.edu/qa/listing.php?id=819
https://www.livescience.com/50678-visible-light.html
http://www.wolframalpha.com/input/?i=(5*10%5E14+hertz)%2F(2450+megahertz)
https://revisionworld.com/gcse-revision/physics/electromagnetic-radiation/radio-waves-microwaves
http://www.schoolphysics.co.uk/age16-19/Wave%20properties/Wave%20properties/text/Microwave_ovens/index.html
https://geekswipe.net/science/physics/why-does-microwaving-a-grape-produce-plasma/
https://www.digikey.com/en/articles/techzone/2017/apr/understanding-antenna-specifications-and-operation
http://searchmobilecomputing.techtarget.com/definition/dipole-antenna
http://www.npr.org/templates/story/story.php?storyId=107010828

Images:

https://commons.wikimedia.org/wiki/File:Starsinthesky.jpg
https://commons.wikimedia.org/wiki/File:Consumer_Reports_-_Kenmore_microwave_oven.tif
https://www.youtube.com/watch?v=hSDpuFgahKY
https://en.wikipedia.org/wiki/File:Dipole_antenna_standing_waves_animation_461x217x150ms.gif
When you think about plasma, you know, like when you’re just sitting around, thinking about states of matter, you probably imagine stars, or the stuff that comes out of a lightsaber.

You probably don’t think about something you’d make in your kitchen microwave. But it’s totally possible, and there are plenty of YouTube videos to prove it.

Thanks to some cool physics, you don’t even need fancy technology. All you need is a microwave that you’re willing to destroy … and a grape. Plasma is kind of like a gas, except that its atoms have some of their electrons stripped off, so it’s made up of positively-charged ions and a bunch of free electrons zooming around.

To make some, you just have to make some gas really hot, and then send an electrical current through it. And a microwave is a great way to do that. Microwave ovens cook food by bathing it in a kind of light called, shockingly, microwaves.

They’re just like the visible light coming out of your computer screen right now, but their frequency, the number of times they oscillate back and forth each second, is about 200,000 times lower, so matter responds differently to them. The waves make water and other molecules in your food vibrate, which generates the heat that makes your hot pocket hot. Now, if you throw a regular old grape in a microwave oven, it will probably just get really hot and explode.

You’ll get fruit juice all over the inside of the thing, but you won’t see any fireballs. To get the fiery ball of plasma, the grape needs some preparation. In those YouTube videos, they’re always careful to cut the grape almost completely in half, except for a tiny strip of skin connecting the two halves.

And that strip is important, because it lets the grape act like an antenna and that makes the whole experiment work. A sliced grape happens to be just about the perfect size to trap and amplify microwaves, and when the waves start flowing in, the grape turns into what’s called a dipole antenna, where the electrical charges in each half get pushed back and forth across the little slice of skin. To make sure all the microwaves hit the grape from the same direction, people will usually take out or cover the spinning plate that rotates the food, so the charges bounce back and forth in the same way over and over again.

All that focuses the energy of these microwaves onto that tiny bit of skin. Any water on there boils away pretty quickly and then, unsurprisingly, the strip catches on fire after just a couple of seconds. Which is one reason why this is a bad idea to try at home: Fire and powerful electronics, not usually a good thing to mix together.

But the fire doesn’t just disappear after that. Hot air rises, so the flame climbs up between the two grape halves. And since the microwave oven is still going, electric current keeps bouncing back and forth between the sides of the grape, and across any steam or hot molecules carried up by the fire.

Eventually, it gets so hot that the atoms in the steam, the air, and even the grape juice have their electrons stripped off, and you get a bunch of plasma. The plasma fireball shoots out from the grape halves and, if everything goes just right, or horribly, horribly wrong, it tends to hit the roof of the microwave often pretty spectacularly. Once that fireball stops and it cools down, the plasma just turns into regular gas again, and there’s nothing to worry about, except, probably, for your broken microwave oven.

Which again, is why this is not a great idea to do in your home microwave oven, which you probably bought with money. So if you’ve seen these grapesplosions on YouTube, there’s a lot more to them than meets the eye, and there’s a lot of cool microwave science out there. But I am not condoning you doing them.

It’s the kind of thing that should be done carefully by professional grape exploders. Thanks for watching! Besides perfectly cut grapes, there are plenty of other things you should not stick in the microwave, and we’ve explained some of them before here on SciShow.

If you’ve ever wondered why you shouldn’t put metal in there, you can watch our episode where we tell you all about it.