I put a 1/4 inch thick clear glass container into the microwave with a plate on top and put it in for almost 5 minutes (there was lots of soup). When It came out the soup was really hot but I could still hold the top parts of the container. After measuring the soup's heat it was about 120-130 degrees but the top part felt cool to the touch. Why didn't it heat up like the soup did?

  • $\begingroup$ Microwave ovens deliver heat to things that are electrically conductive, like water that has stuff dissolved in it, which is almost all water. Glass is an insulator, so it doesn't get heated (by the microwaves, it may get heated by the soup). Never put metal objects in a microwave. They really get hot! $\endgroup$ Aug 20 '16 at 22:03
  • $\begingroup$ Microwave ovens deliver heat to things that are electrically conductive - why? $\endgroup$
    – EasyPeasy
    Aug 20 '16 at 22:08
  • $\begingroup$ Because microwaves are radio waves, and anything that conducts electricity functions as an antenna. The current induced in the conductor makes it hot. Electrical transformers work almost the same way. Back when we had distant early warning radars in the arctic, workers could warm up by standing in front of them. $\endgroup$ Aug 20 '16 at 22:17
  • $\begingroup$ @Mike Those should be answers, not comments. $\endgroup$
    – rob
    Aug 20 '16 at 22:26

The microwaves are primarily designed to vibrate/heat the water molecules in the food, as a way of ensuring that the foods gets cooked evenly. An aid to this process is the rotating plate within the machine.

Microwaves that cook your food pass through plastics, glass, and ceramics, with mimimal heating, as their water content is low and they are less prone to heating, explaining why you can pop your (almost ready to go) chicken curry and rice, along with its plastic packaging, straight into the microwave. It is also this feature of microwaves that makes them so energy efficient; they heat only the food and nothing more.

However, don't try to put eggs in a microwave, they will become minibombs as the water heats up, turns to steam and then blows the eggshell apart. Cups of water are not recommended, nor is just pressing the start button without food or liquid to absorb the microwaves, as the magnetron (which is what cooks your food), ends up absorbing the microwaves instead, which can damage your microwave and may even start a fire.

Metals, on the other hand, reflect these radio waves, a characteristic very cleverly put to use in the walls of the microwave such that no waves escape and cook anyone in the kitchen! However, you can see sparks from the edge of some decorative ceramic plates appearing now and again, from the microwave radiation.

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Image Source: www.ccohs.ca

  • $\begingroup$ It's quite relative though. I use 800 W MW to preheat china plates (prior to serving food on them) all the time: 2 'standard' plates for 1 min get to 40 - 50 C easily. I also use a MW occasionally to dry empty lab glass ware with. Same effect. $\endgroup$
    – Gert
    Aug 21 '16 at 0:10
  • $\begingroup$ @Gert Looking up the Web for the above answer, I came across microwave a chocolate bar and see the wavelengths appear, but have I the willpower? chocolate mmmm......... I do think the plates heat up as well, but apparantly heated plastics might leech god knows what into the food, so plates from now on. $\endgroup$
    – user108787
    Aug 21 '16 at 0:31
  • $\begingroup$ Plastics like PE or PP absorb very little MW though. They are also quite 'pure', clear ones containing at most a bit of anti-oxidant/anti-UV/dye. MW absorbance is linked also to molecular polarity, i.e. permanent dipoles (own research!): PVC/PET/ABS show much higher absorbance than PE/PP, for example. $\endgroup$
    – Gert
    Aug 21 '16 at 0:58

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