Microwave Oven Physics
A microwave oven is an applied-physics machine: it floods a metal box with electromagnetic waves tuned to jiggle water molecules. The hot and cold spots in your leftovers are a standing-wave pattern you can measure with cheese.
Dielectric heating
Water molecules are electric dipoles. The oven's 2.45 GHz field flips direction 2.45 billion times per second, forcing the dipoles to rotate back and forth. Molecular friction from this frantic rotation becomes heat — inside the food, not just on its surface.
Why 2.45 GHz
This frequency is not the resonant frequency of water (a common myth) — it is an ISM band chosen to avoid interfering with communications, and it penetrates food a few centimeters deep. True resonance would heat only a thin outer layer.
Standing waves and the turntable
Microwaves reflect off the metal walls and interfere with themselves, forming a standing-wave pattern with hot nodes and cold antinodes spaced about 6 cm apart (half the 12.2 cm wavelength). The turntable's entire job is dragging food through the hot spots.
The Faraday cage door
The metal mesh on the door has holes far smaller than the 12 cm wavelength, so the waves cannot escape — a Faraday cage. Visible light, with its sub-micrometer wavelength, passes through easily, which is why you can watch your food.
Try this yourself
- Remove the turntable, cover the floor with sliced cheese, run 20 seconds, and measure the distance between melted spots — it should be ~6 cm, half the wavelength.
- Put a phone inside (do NOT turn the oven on) and call it — then try with the door mesh covered. The cage blocks the microwaves but not all phone bands.
- Notice which containers stay cool: microwave-transparent materials have no dipoles for the field to grab.