This video is one that is incredibly interesting, but also quite difficult to explain simply. My warning – unless you’re prepared to risk your microwave being damaged beyond repair, don’t try this at home! Please also be warned that there is some coarse language in the video below.
Here is an attempt at an explanation; but just remember, I’m not a physics expert so this is just my best guess. Firstly, the way that microwave ovens work is that a magnetron inside the microwave produces a large quantity of electromagnetic radiation in the microwave region of the spectrum (where the wavelength is about 12.5 cm in air). These microwaves waves have less energy than visible light (so the idea that microwaves are hazardous is nonsense) but they just so happen to be at the right frequency to make water molecules vibrate more vigorously inside an object (such as food). These vibrations cause heat to be released, heating up whatever object is inside the microwave – such as a grape, or last night’s leftovers!
The shape of the grape, especially when cut almost completely in half, means that the microwaves are amplified inside the grape, making it act like an antenna. All this extra energy inside the grape means that the water inside the grape becomes superheated (i.e. to a temperature above 100 degrees) and escapes violently. As best as I can tell, the reason that a plasma is created is because the water inside the grape is acidic and not pure, which means that it contains ions that conduct electricity. When it escapes as superheated steam, the ions that are dissolved in the water allow an arc to be created between the water droplets, turning the steam into a plasma (which is essentially a gas with an electric charge) which emits a large amount of light. Phew!
Because the steam is what is causing the arcing effect, it needs to be contained under the glass, otherwise it will just dissipate and the arc will fizzle out. I can’t completely explain why there appear to be flames when he first microwaves the grape; perhaps the sugars in the grape combust because of the heat?