Why a microwave heats some spots more than others
Inside a microwave oven, metal walls reflect the microwaves, so the waves cross and overlap in the cooking space. In some places they reinforce one another; in others they partly cancel. The resulting pattern of stronger and weaker energy is called a standing wave. A portion of food sitting in a weak-energy spot can warm much more slowly than a portion only a few centimeters away.
Food also absorbs microwave energy unevenly. Water, salt, and other ingredients affect how readily a particular patch absorbs it, while the food’s shape and thickness change how the waves reach its interior. Microwaves can penetrate into food, but they do not heat every point equally or make heat spread instantly through a dense portion.
Why the middle can stay cold
A thick mound of leftovers can have warm areas near its outside while its interior remains cool. Some microwave energy is absorbed along the way in, and heat then has to travel from warmer regions toward cooler ones by conduction—the slow movement of heat through matter. That takes time, especially in a compact serving. Meanwhile, the food may also be positioned in a weak part of the wave pattern.
The oven’s turntable helps by moving the food through different parts of that pattern. It reduces persistent hot and cold spots, but cannot eliminate them: a dish may be off-center, oddly shaped, or too large for rotation to even out its heating.
A pause is part of the heating
Stirring or rearranging food moves cold portions into warmer positions, and letting it stand briefly gives heat time to spread inward. For a thick serving, warming in shorter intervals with a stir between them can be more effective than one uninterrupted run. The microwave supplies energy unevenly; the pause and the stir help the food redistribute it.
