Microwave Ovens Heat Food Unevenly
Constructive and Destructive Interference of Electromagnetic Waves
Nov 11, 2007
What Are Microwaves?
Microwaves, like other radio waves, are a form of electromagnetic waves. Electromagnetic waves are wavelike oscillations of electric and magnetic fields. Electric fields are what makes electric charges attract or repel. Positive or negative electric charges produce electric fields which in turn act on other charges. In a similar way, magnetic fields cause magnetic forces.
These fields are perpendicular to each other and continually oscillate between maximum positive and maximum negative (pointing in the opposite direction). The microwaves used to heat food in microwave ovens have a wavelength of 12.2 cm and oscillate at a frequency of 2.45 gigahertz. Giga means billion, so the electric and magnetic fields oscillate fast enough to make 2.45 billion complete cycles each second.
Microwaves heat the water in food. Foods with different amounts of water absorb microwave energy with different efficiencies causing uneven microwave heating. In ice the molecules can not rotate as easily, so microwaves do not heat ice as effectively as they heat liquid water. The heated liquid water must heat the frozen water by normal conduction. So frozen foods can heat unevenly.
However constructive or destructive interference can cause uneven heating even in uniform unfrozen foods.
What is Constructive and Destructive Interference?
When two or more electromagnetic waves propagate past the same point, they interfere with each other. This interference can be constructive or destructive interference. In constructive interference the effects of the two waves add, so the resulting wave has a greater intensity. In destructive interference the effects of two waves subtract, so the waves cancel each other out. The intensity is zero or very small.
For electromagnetic waves, the peaks of the waves occur when the electric and magnetic fields are maximally positive. The valleys occur when these fields are maximally negative.
If two microwaves are interfering so that the peaks and valleys of one wave align with the peaks and valleys of the other wave, then the combined electric field is twice as intense as the electric field from a single microwave. The greater intensity means constructively interfering microwaves have more energy to heat food. The portion of your meal at that position gets very hot.
If two microwaves are interfering so that the peaks of one wave line up with the valleys of the other wave, then the electric fields from the two waves cancel each other out. One is positive and the other is negative, so they add to zero. The net intensity of the microwaves is zero, so there is no energy available to heat the food. The portion of your meal at that location stays cold.
The microwaves in a microwave oven originate from a magnetron. Once inside the oven chamber they reflect off the metal interior walls. So the microwaves bounce around several times inside the oven before they are absorbed by the food. Hence, at every point inside the microwave oven, there are several microwaves interfering with each other. Where this interference is constructive the food gets scalding hot. At other nearby points the interference is destructive, and the food stays cold. The microwaves used in microwave ovens have a wavelength of 12.2 centimeters. (2.54 centimeters = 1 inch.) So the points where the food is hot and where the food is cold are typically a few centimeters apart.
Rotating turntables in microwave ovens move the food around to minimize the uneven heating. Microwave ovens without a rotating turntable should be stopped halfway through the cooking process to rotate the food manually.
Further Reading
Bloomfield, Louis A., How Things Work The Physics of Everyday Life, Wiley, 1997.
