Prof. Tai Kai NG and Mr. Chi Wai CHAN
Department of Physics, Hong Kong University of Science and Technology

Microwaves form parts of the electromagnetic spectrum with typical wavelengths from 1 millimetre to 10 centimetres – something in between light waves and radio waves (Figure 1). Like any other electromagnetic waves, the microwave is composed of both electric and magnetic fields, which are perpendicular to each other and propagates at the speed of light throughout space(speed of light “c” in vacuum is roughly 300,000,000 metres per second, or 300,000 kilometres per second) . Apart from ovens, microwaves are also used in telecommunications, e.g., radars, wireless computer networks and mobile phones. In fact, the entire universe is filled with microwave radiation left by the Big Bang Explosion of the early Universe.

(Figure1 The Electromagnetic Spectrum)

The core part of a microwave oven is the microwave generator. In the early designs, the generator is a magnetron, which is a vacuum tube that converts electrical energy into microwave energy. The microwaves are transported to the cooking chamber by a wave-guide, analogous to electrical wires transporting electricity. The cooking chamber is designed to keep the microwaves, so that microwaves bounce off the walls like beams of light in a mirrored room around and absorbed by food. Metal wiring on the glass window of the door keeps the microwaves from leaving the cooking chamber.

(Microwave Oven)

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 by microwaves. Water molecules are polar molecules with one end dominated by a negative charge and the other by a positive charge. The typical frequency of the microwave produced by a microwave oven is of the order of 2.5 GHz (1 GHz = 1,000,000,000 Hz). That is to say, the electric fields produced by the magnetron oscillate back and forth at the rate of 2.5 billion times per second (defining 1 billion = 1,000,000,000). Since the water molecules are polarized, the water molecules in the food will also get pulled back and forth at the rate of about 2.5 billion times per second. This rapid back-and-forth motion between water molecules creates friction, and hence heat. Typically, microwave can only penetrate about 3.5 to 5 cm into the food. The centre of the food is cooked mainly by heat conduction. The hot and cold spots on the food are caused by the interference effect of microwaves, similar to bright and dark spots on a screen caused by interference of light after passing through a grating. In order for microwaves to distribute evenly around the food, a turntable or a rotating source is often used.

(Cooking with mcirowave oven)

The exact amount of microwave energy need to cook a piece of food depends on its properties. In practice, the amount of microwave energy get absorbed and reflected can be measured for each food substance. Food substance contains a substantial amount of water is a good absorber of microwave energy. When water is changed to a different state, such as ice, it becomes a poor absorber of microwave energy. Other factors determining the effectiveness of microwave cooking are the shape and material of the container used. Round containers generally have better cooking result as food tends to be overheated at sharp corners and edges.

Microwave radiation can pass through plastic and glass, but not metallic objects. This is why the glass window of the door of microwave oven is laced with metal wiring; the metal wiring keeps the microwaves from leaving the cooking chamber. If you put a metal object, such as a fork, into the oven, microwaves hitting the fork will get reflected back to the source. The extent to which microwaves are reflected varies from the types of metal. Certain metal composites actually absorb microwave energy.

Let us take a closer look at what happens when microwaves come into contact with metallic objects. When microwaves are reflected from a metallic object, it produces a so-called arcing effect (Arcing is a microwave term for sparks in the oven). Arcing is resulted from the build-up of excessive charges in metallic materials. In microwave ovens, this can occur when two or more metallic objects, e.g., a metallic food container and the inner metallic wall, are placed close to each other causing the air between them to ionize, producing sparks. The arcing effect can also take place when a cooking utensil with sharp metal edges or burnt food are come into contact with microwaves inside the oven. If the arcing object is further bombarded by microwaves, the temperature of the object rises sharply and being overheated and eventually causing a fire in the oven.

A phenomenon known as superheating 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 100 ). The superheated state is an unstable state. When water is superheated, the presence of an external agent such as a spoon or even milk powder can cause the water to boil vigorously into explosion. If one litre of water is superheated by only 1 , it can produce about 3 litres of steam. You can see how dangerous it is.

The safety of microwave radiation in microwave ovens is still a subject of intense research. From the above discussions it is clear that we should not put metallic objects or boiled water into microwave oven. Other tests have revealed that nearly all microwave ovens in the market emit microwaves whilst in operation. In general, the amount of microwave emission decreases with the distance. The official recommended value of microwave emission for all domestic ovens is about .

As cellular or mobile phones are getting increasingly popular in recent years, the effect of microwaves on our health has also been a subject of study. The main concern is the effect of heat produced by microwave radiation on our body, particularly the brain. Despite the fact that the recorded temperature rise on our body caused by microwave radiation emitted from mobile phones was so low, just a fraction of a degree Celsius, some researchers believe that radiation emitted from mobile phones could increase the risk of brain diseases such as the Alzheimer's disease.

In addition to microwave radiation, micro-waved foods are also subject to investigation. 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.