Answer:  

A physics professor kindly answered about “electric waves”, “why metal reflects electric waves” and “why it is dangerous to put metal into microwaves (from the view of physics)”. Sharp Corporation kindly answered about “why it is dangerous to put metal into microwaves (from the view of manufacture)”.
 
About electric waves
Electric waves, which are used for TV, radio, cell phone signals, and so on, is a kind of alternating current. The electric wave is called short wave, ultrashort wave, micro wave, etc, according to its frequency (the number of vibrations). When alternating current is of low frequency, it can not travel (and therefore conduct) without electric wire. The frequency of electricity for household use is 60 Hz or 50 Hz (in Japan). This frequency is so low that wire is necessary in order for the electricity to travel. When frequency is high, the electric wave can travel in the air without wire. Therefore, we can use TVs, radios, and cell phones without wire (i.e. wireless). However, the electric wave traveling in air is disturbed by obstacles. In particular, when the obstacle is metal, this metal causes the electric wave to bounce. Thus, it is necessary to use an antenna to guide electric waves inside reinforced concrete buildings or underground shopping centers. The viewport of a microwave is designed to prevent the electric wave from leaking outside.
 
Why do electric waves reflect from metal?
By the way, why are electric wave reflected from metal? One of the major properties of metals is that they are packed with electrons which can move freely. Because the electrons can carry current, metal is good conductor of electricity. The same thing happens in the case of alternating current. When electric waves hit the surface of metals, electrons near the surface of  these metals are given the order to move. Since there are so many electrons on the surface, the electrons which receive the order from the electric wave move from side to side, thus preventing the electric wave from entering the metal (shielding effect). Thus, the electric wave is obliged to return. This is the reflection of an electric wave.  

Some electrons which receive the order from the electric wave move too much, causing the electrons jump from inside metal into air.  When the electrons which jump into air move to something which conducts electricity, this motion produces light (spark) or sound (pop!). This is electronic discharge.

    
Why is it dangerous to warm up metal by microwave? (from the physics point of view)
There is an instrument called a magnetron, which generates strong electric waves. The frequency of these waves is 2450MHz, which means that the wave vibrates 2.5 billion times per second. If metal is put inside microwave, the electronic discharge happens so intensively because electrons inside metal are accelerated by the strong electric wave. This also causes a bad influence on the microwave itself. There is sometimes thin metallic decoration on the surface of some coffee cups and dishes. Since the metal is so thin, we can think that all of the metal is surface. In such a case, all of the electrons in the metal run around so that abnormally high heat is generated, causing the metal itself to burn or melt.
 
Incidentally, water molecules have hardly any electrons which can move freely. However, because the whole molecule is shaken effectively (resonance) by the electric wave generated by a magnetron (this vibration is heat), water is heated up. The frequency of this vibration is just 2450MHz.
 
Why is it dangerous to warm up metal by microwave? (from the manufacturer point of view)
The following are the reasons from the manufacturer point of view why we ask users not to put metal in the microwave:

1. If a metallic container is heated up in a microwave, many reflected waves are generated by the metal. The reflected waves return to the antenna of a magnetron, causing an increase in temperature. This causes the deterioration of the magnetron and generates electronic discharge easily, resulting in microwave failure. 
    

2. If a metallic container is close to the wall of a microwave, electronic discharge occurs between the wall and the metallic container, causing microwave to develop problems. Also, if this happens, users are surprised at the noise and the light, then they complain to the manufacturer. And so, we manufacturers prohibit the use of metallic containers in microwaves.

   
Acknowledgement
We would like to thank an anonymous physics professor and Sharp Corporation for their kind answer.
 
 

This article is translated by Chemistryquestion.com from the original article in Chemistryquestion.jp.  Please let us know if you find any errors.