Waves that overlap themselves
Why Does a Microwave Heat Unevenly?
The waves bounce off the walls and cross each other, making hot lines and cold lines.
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Step 1 of 5
Microwaves shake water molecules.
The waves flip direction billions of times a second, and water molecules twist to follow. That twisting is what heats the food.
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Step 2 of 5
The metal box reflects every wave.
The walls and the mesh in the door bounce the waves back inside, so they cross the oven many times instead of escaping.
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Step 3 of 5
Crossing waves add and cancel.
Where two waves peak together the effect doubles. Where a peak meets a trough they cancel to nothing. Both patterns stay in fixed places.
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Step 4 of 5
The spots sit about six centimetres apart.
That spacing is set by the wavelength, so hot and cold bands are always roughly that far apart — which is why one bite scalds and the next is icy.
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Step 5 of 5
The turntable and standing time fix it.
Rotating drags every part of the food through the hot bands, and resting afterwards lets heat spread by conduction into what the waves missed.
You can map the spots by microwaving a tray of marshmallows and measuring the gaps between the melted patches.
The short version
Microwaves reflect off the metal box and overlap, making fixed hot and cold bands that the turntable and standing time even out.
Try it yourself
Heat a plate of rice without turning it, then feel across it. The warm and cool bands are surprisingly regular.
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