The plasma formed between two hydrogel beads glows as they are irradiated in a microwave oven.

A bright plasma flares as a microwave heats two beads, replicating the effect of microwaving a grape. Credit: Hamza K. Khattak

Physics

Why microwaving a grape sparks a fiery glow

Scientists debunk a popular explanation for a flashy party trick.

The physics of light scattering off spheres helps explain why a grape heated in a microwave oven can generate a brilliant, highly charged gas.

In dozens of YouTube videos of microwaved grapes, a plasma — a glowing gas of charged particles — flares from the skin ‘bridge’ that holds together the two halves of a severed grape. According to many online explanations, the grape halves act as an antenna tuned to the energetic microwaves. Because the moist skin is highly conductive, a current flows through this antenna and along the surface of the bridge.

Aaron Slepkov at Trent University in Peterborough, Canada, and his colleagues found that this explanation is incorrect. Using thermal imaging and computer simulations, they showed that the grape halves form a cavity that absorbs and focuses the microwave radiation into a hotspot where the halves touch each other. This radiation ionizes potassium and sodium atoms in the grape skin.

The team microwaved twin beads — which were composed of nearly pure water, with no skin — that were in contact with each other. The resulting hotspot at the beads’ point of contact was capable of generating plasma.