I think you should be more respectful when shooting off your mouth (fingers) on the keyboard. You have no idea of the qualifications or background of the person you are commenting to. Maybe it can be someone who is a specialist, and has worked in the design of these things!
I think you should have respect for others. You should have asked about why you can put a metal rack in some microwave ovens, and not put someone down because of your pure ignorance!
The only reason I am answering this, is because there are many others who have some respect, and would be interested in the answer.
If you were educated in the field of physics, mechanics, or electronics, you would realize that there is something called resonance. Microwave ovens work in a particular frequency band, and therefore metal objects can be designed to not react to the frequency used in the oven. The metal cavity that the oven is constructed with, is invisible to the RF (Radio Frequency) energy being emitted from the magnetron. This metal cavity is actually a non terminated waveguide, and thus anything that is out of the exact resonance will become a load to it, and be heated up. The metal rack is designed in size and in the way it is formed to act as a near zero load to the wavelength (frequency) used. Even the paint used inside of the microwave oven must have specific characteristics. Because the cavity of the microwave oven is non terminated, is reason why the magnetron can be easily damaged if the oven is used without cooking something inside. Even the location and type of lamp used for the illumination in the microwave oven is critical.
The standard frequency used in the home cooking microwave oven is 2.45 gHz. This frequency was not pulled from rolling dice. There are a number of reasons for this choice.
Radio waves in this frequency range have a particular property where they are absorbed by water, sugars, and fats. When they are absorbed they are converted directly into heat by inducing accelerated vibration of the molecules. Microwaves in this frequency range have another interesting property. They are not absorbed by most plastics, glass, or ceramics.
Only the molecules that are being excited are heated up. This way non reacting containers will not heat up. The container of the food being cooked will only heat up by conduction from the heated food inside.
Metal reflects microwaves, which is why metal pans do not work well in a microwave oven, and can even cause damage to the magnetron by overloading it. Under some conditions, metal can conduct the microwaves and act as an induced conductor, thus converting the energy back to electricity. This is the reason why some metal object will cause sparks, and even make things catch on fire. The only exception to this is if the metal object can be in resonance to the wavelength to not be seen as a load.
There are industrial microwave type ovens that are working at other frequencies. These are used for drying specialized epoxies, curing plastics, and many other applications. There are also microwave dryers used in some types of fabric manufacturing processes. Some of these are working down at
1.5 gHz, and others are working up to about 3.5 gHz, depending on the application. These will not be good for cooking foods.
If you have one of these 2.4 gHz telephones it is normal that when you go near to your microwave oven while it is working, the phone may be interfered with. This not dangerous. There is a slight amount of energy leakage from all microwave ovens. There are specifications for this leakage. It must not exceed a particular amount.
The limit in most countries is 5 milliwatts of microwave radiation per square centimetre at approximately 2 inches from the oven surface. This is far below the level known to harm people. As you move away from an oven, the level of any leaking microwave radiation that might be reaching you decreases dramatically. For example, someone standing 20 inches from an oven would receive approximately one one-hundredth of the amount of microwaves re ceived at 2 inches. The energy drop-off works according to the inverse-square-law factor. Go to
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for more details about microwave oven safety and applications.
We recommend that children and pregnant women should walk away from the microwave oven while it is working. We recommend about 6 to 10 feet. This will dramatically reduce any risk of radiation absorption. There have been many studies about low level radiation from microwave ovens, TV sets, monitors, and other home appliances. There have not been anything conclusive as of yet, so it is better to not take any chances. It is known that over certain intensities, electromagnetic and RF radiation is dangerous, and will cause cancer, and other types of biological damage. The dangerous intensity level will be different at different frequencies. This is a complex field in itself. Caution is the best advice. Harmful radiation is not able to be felt, unless it is very intense. The effect of this radiation can be accumulative if the exposer is frequent.
If you want to be very safe, you should have your microwave oven checked at least once a year. The door seal and hinges can wear out. This is the greatest weakness in microwave ovens. It only takes a few thousandths of an inch of extra space to allow higher radiation leakage. The service centres that service microwave ovens should have the proper instrument to measure the microwave radiation. Some places will not charge to take the measurement. Others may charge a few dollars for the technician's time. If there is leakage, this can usually be easily repaired. Sometimes it is just the hinge adjustments, or replacing some simple parts.
We do not recommend that the consumer attempt to service his own microwave oven. It is the most dangerous appliance in the home if it is not used properly, or is opened up. The TV set and computer monitor is the second runner up to this.