1. Microwaves and Microwave Oven

2. History
The idea of microwave oven was first conceived by Percy L. Spencer in 1946 and finally patented in 1950.
Mechanism：the principle of frictional heat production using microwaves.
The early ovens are huge--weighted over 300 kg and measured over 1.5 metre in height--were mainly used in large restaurants and food outlets.
The first home version was produced by Raytheon Corporation in 1965.

3. What are Microwaves？
Microwaves are electromagnetic waves which consists of both electric and magnetic fields perpendicular to each other and propagates at the speed of light.
Microwaves form parts of the electromagnetic spectrum with typical wavelengths from 1 millimetre to 10 centimetres – something in between light waves and radio waves.
Microwaves are also used in telecommunications, e.g., radars, wireless computer networks and mobile phones.
The entire universe is filled with microwave radiation left by the Big Bang explosion of the early Universe.

5. A domestic microwave oven
The Microwave Oven
A domestic microwave oven
The microwave generator is a magnetron.
Magnetron converts electrical energy into microwave energy. Microwaves produced are transported to the cooking chamber by a wave-guide.
The cooking chamber is designed to contain the microwave, so that microwave bounces off the walls like beams of light in a mirrored room around.
Metal wiring in the glass window of the door keeps the microwaves from leaving the cooking chamber.

6. Cooking with Microwaves
Microwave cooking is a process of exciting the water molecules in food. Food substance that does not contain water, such as oil, usually does not get cooked in a microwave.
The polarized water molecules in the food will get pulled back and forth at the rate of about 2.5 billion times per second by the electric fields of the microwaves.
This rapid back-and-forth motion between the water molecules creates friction, and hence heat.

7. Microwave can only penetrate about 3. 5 to 5 cm into the food
Microwave can only penetrate about 3.5 to 5 cm into the food. The centre of the food is mainly cooked by heat conduction.
The interference effect of the microwaves produces the hot and cold spots on the food，a turntable or a rotating source is often used to distribute evenly around the food,

8. Cooking with Microwaves
And then the heat is transferred to the molecules of food nearby.
This rapid back-and-forth motion between the water molecules creates friction, and hence heat.
Magnetron produces microwaves which pull the water molecules back and forth at the rate of about 2.5 billion times per second.
represents water molecule;
represents molecules of food.

9. The exact amount of microwave energy need to cook a piece of food depends on its properties.
Food substance containing a substantial amount of water is a good absorber of microwave energy.
The effectiveness of microwave cooking are determined by the shape and material composite of the container used. Round containers are generally more suitable.

10. Metallic Objects
Microwave radiation can pass through plastic and glass, but not metallic objects. This is why the glass window of the doors of a microwave is laced with metal wiring.
Metals reflect microwaves. When microwaves are reflected from a metallic object, it produces a so-called arcing effect. Arcing results from the build-up of excessive charges in metallic materials.
When two or more metallic objects, e.g., a metallic food container and the metallic interior of the wall, are placed close to each other causing the air between them to ionize, producing sparks.
If the arcing object is further bombarded by microwaves, the temperature of the object rises sharply and overheats and eventually causing a fire in the oven.

11. Superheating
It is known to occur when a cup of water is heated in a microwave oven.
The water is referred to as superheated when its temperature increases above its normal boiling point (the boiling point for water at standard atmospheric pressure is 100oC ).
The superheated state is an unstable state.
The presence of an external agent such as a spoon or even milk powder can cause the superheated water to boil vigorously into explosion.
If one litre of water is superheated by only 1oC, it can produce about 3 litres of steam.

12. Safety of Microwaves
Metallic objects or boil water should not put in microwave oven.
Other tests have revealed that nearly all microwave ovens in the market emit microwaves whilst in operation.
Mobile phones：Will the heat produced by microwave radiation affect our body?
Micro-waved food：Some tests have revealed that the molecular structures of nutrient in vegetables, such as carrots and broccoli, are being deformed at cellular level by high frequency microwaves.

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