Grape Balls of Fire

What happens when you cut a grape almost in half, cover it with a glass, and microwave it?

Sparks.  Big sparks.  Just watch this video (and if you want to fast forward to the sparks, they start the 1:43 mark or thereabouts):

So, what’s happening here?

The two sides of the grape act as focal points for the microwaves (the waves themselves, not the appliance).  The grape halves are connected only by the thin piece of skin left uncut by the knife.  As the microwaves move across the grape, from one hemisphere to the other, this tiny remainder of grape skin quickly dries out and burns up, causing a spark.

And one spark is all you need.  It ionizes the air around the grape, creating ion-rich gas also known as a plasma — with solids, liquids, and gasses, the fourth form of matter.  The light show you’re seeing is the plasma, much like that as seen in a plasma lamp, albeit more violent and because it is in your microwave, dangerous.

Why is the plasma in the video above so large?  The team at The Naked Scientists explains further: “This plasma conducts electricity and can absorb microwaves. Sometimes the plasma gets big enough to absorb enough microwaves to keep growing[.]”  The Naked Scientists also warn that the experiment “can cause minor burns on the top of your microwave.”

The glass contains the air around the grape even more so than the microwave oven, thereby concentrating the plasma, and allowing for the light show seen in the video (as well as protecting the roof of the microwave).


Bonus fact: Ivory soap does something neat when microwave as well — as seen here.

From the Archives: Zapped Chocolate: How the power of microwaves was unexpectedly discovered.

Take the Quiz: Name the countries that produce the most grapes​.

Related: You can’t buy grapes on Amazon, but you can buy Ivory soap.

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