Like light and radio waves, microwave also is a form of electromagnetic radiation. Microwaves are used mainly in the field of telecommunications, radar, cooking, and some medical treatments and for transmitting satellite television signals.
Quite by accident Dr. Percy Spencer, an engineer with Raytheon Corporation discovered that microwaves could heat foods, when a chocolate bar melted in his pocket while he was working near microwaves.
The microwaves used to cook food have a frequency of about 2450 Megahertz i.e. 2.45 Gigahertz. At this frequency microwaves have several interesting properties. Water, fats and sugars readily absorb these microwaves and get heated up quickly. Metals however reflect the microwaves back and this keeps the microwaves within the oven chamber. Most types of glass and plastic do not absorb microwaves but simply allow the microwaves to pass through without heating the glass or plastic.
Since almost all food items contain water, it is easy to use microwaves to heat the food, without heating the container, the air surrounding the food or the oven itself.
To understand the significance of this property of microwaves consider the example of boiling potatoes on a regular gas stove and in a microwave oven.
To do this on a gas stove, you would have to put the potatoes in a vessel filled with water and heat it. The heat is first transferred to the vessel, then to the water in the vessel and from there to the potatoes.
In the microwave, simply put the potatoes on a suitable plate, and run the microwave for about 8 to 10 minutes. All the microwaves are absorbed by the water molecules in the potatoes causing them to heat up from inside. As a result you have boiled potatoes quickly. No energy is wasted in the container or the surroundings. The potatoes are cooked by directly heating the water that is present inside them.
Since cooking always destroys some of the nutrients in the food, the short cooking times in the microwave mean that the food is more nutritious as compared to food cooked by other means.
However the fact that the surroundings are not heated means that the cooked food does not get the crisp brown look like food cooked in an ordinary oven. Many people feel that this causes a big difference in the taste.
The most important part of the microwave is the magnetron whose function is to generate the microwaves. Electronic circuits are used to provide the high voltage for the magnetron, the timer for controlling the cooking time and the door interlock for preventing the door from being opened while cooking is in progress.