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Microwave ovens use electromagnetic waves to heat food.

Most food items contain polar molecules like water, that flip back and forth with the alternating electric field. While flipping, the molecules undergo friction with other molecules, as a result, heat is generated.

The microwave chamber acts as a resonating cavity where standing electromagnetic waves are formed by reflection from the metal surface. The two metallic surfaces are placed at nodal planes.

So, the distance between the planes must be an integral multiple of half the wavelength.

The wavelengths that satisfy this condition are called the characteristic wavelengths, and the corresponding frequencies are called the characteristic frequencies.

Each characteristic frequency, wave shape, and node pattern constitutes a set of normal modes.

For instance, a typical domestic microwave runs at 2.4 gigahertz, corresponding to a wavelength readily absorbed by the water molecules.

This wavelength has nodes at 6.1 centimeters apart, which means that the food will be cold at these points and heated to a maximum at the antinodes.

So, the microwave's turntable is rotated to have a uniform heat distribution.