How do microwave ovens cook food?
Microwave ovens are actually radar sets. Radar was developed prior to WWII in both England and the United States and is credited with giving the British a fighting chance in the Battle of Britain in 1940-41. Those early radar workers sometimes got quite a surprise. When radar technicians moved in front of a working radar antenna, the candy bars in their pockets would melt.
The heart of a microwave oven is fist-sized vacuum tube called the magnetron. A magnetron is an electronic device that creates electromagnetic waves by using electricity to heat a filament wire. The resulting electrons are caused to wiggle and emit waves of about 2450 MHz. Microwaves are exactly like light waves, except you can’t see them. Each wave is about five inches in length and too long for the eye to detect. In the microwave oven the beam of waves strikes a fan that distributes the waves evenly throughout the oven.
Most foods that need cooking or heating contain a lot of water. Water molecules are composed of bipolar hydrogen and oxygen atoms. The oxygen atom is slightly negative and the hydrogen atom is slightly positive. When the water molecule is struck by microwaves, it is made to vibrate wildly and rapidly back and forth, rotating first one way and then another. This rotation happens millions of times each second. All this twisting causes friction that heats up the food.
It’s a different story for the dishes. The molecules in dishes contain very few water molecules, so there are practically no water molecules to twist or rotate and cause friction. Dishes, made of paper, glass, ceramic, and plastics do not contain those polar molecules. Most of the heat that the dishes get comes from the food that is being heated. So the dishes in a microwave do not get very hot.
The waves from a microwave oven can penetrate to a depth of about two centimeters or one inch. So the amount of microwave radiation reaching the center of a slab of meat from all sides is more than is absorbed by the an outside layer. This is another of saying that microwaves cook from the inside out. The center of a steak can “get done” while the outside is still pinkish. Traditional gas or electric ovens heat by conduction, which means the outer part is cooked first and the interior is cooked last.
Why no metal in a microwave oven? Metals reflect waves, just like mirrors. Remember, microwaves are the same as radar waves used by police and troopers to catch speeders. Those radar waves bounce off my car (I’ve gotten six speeding tickets to prove it) and return to the police radar receiver.
In a microwave oven, the radar waves keep bouncing from metal piece to metal piece, building up to an alarming spark-producing degree. The excess energy bleeds off and ionizes the air. That’s those tiny bluish lightning bolts you see coming off that errant fork!