Water Molecules React to Frequency of Microwaves

A singular property of water molecules is that they get excited (i.e. start moving) when hit with microwaves. Microwave ovens use this knowledge about water to cook food.


Microwaves

Light Waves
Source - chemwiki.ucdavis.edu

Microwaves are light waves with a wavelength of between 1 mm and 1 meter.
They carry a lot less energy than radio waves and are much less lethal than X-rays.
The frequency of microwaves is just right for water molecules, which absorb the energy in microwaves.
This absorption raises the temperature of water. Microwaves can raise water temperature to water's boiling point.
Microwave Ovens

A Microwave Oven
Source - globalmicrowave.org

These ovens raise the temperature of water in the food placed in the oven, and the heated water helps cook the food.
Microwaves are generally not strong enough to raise temperature of food molecules.
Microwave ovens are designed to spread microwaves evenly all over the food so that waves supply energy distributed equally within the volume of an oven.
Unevenly Cooked or Heated food
A common complaint about microwave oven cooking is that the food we heat or cook in it does not heat or cook evenly throughout. 
Water is the cause of uneven heating and cooking.
As microwaves only heat up water molecules, if these water molecules are not evenly dispersed in the food, the food heats up differently in different places, for example:
- Microwaved soup has a hot layer on top and a cold center. The hot layer on top is the result of the uppermost water particles getting heated the most because they absorb much of the microwave energy before the waves can penetrate deeper into the body of the soup.
- Sometimes the middle of food is cooked "first" in a microwave oven because there is a larger concentration of water molecules in the middle of many foods. The moist center heats up first and longer.
- Foods, dry of moisture, like cornflakes rice or pasta are difficult to cook in a microwave because these foods have little moisture that microwaves can heat up.
- Breads get hard in a microwave oven because microwaves evaporates the little moisture contained in bread and, thus, dry the bread into a hard crust.
Invention of the Microwave Oven

Dr Percy Spencer
Source - Gallawa.com

The original name for microwave ovens was Radarange.

An engineer, Dr Percy Spencer, discovered the impact of microwaves while he was developing radar technology that converts electricity into microwaves.
Dr. Spencer's employer, Raytheon, commercialized microwave ovens and sold the first one in 1947.
Today, 90 percent of US homes have a microwave oven.