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Please forgive my amateur physics knowledge.

Say it's Thanksgiving leftover day. You load a plate with turkey, potatoes, cranberry sauce, and salad. Now it's time to heat your food, but oh no! You put the cold foods on your plate before using the microwave!

The mesh grate over the microwave window reflects microwaves without being damaged. Could you safely place a similar mesh dome over your cranberry sauce and salad to heat half your food, but leave the other half cool?

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If the mesh were to reflect the microwaves, then yes you could heat up one half of a plate and keep the other cold. However, what typically happens is that the microwaves excite electrons in the metal mesh itself, generating eddy currents and sparks. Depending on your microwave, it can even damage the microwave itself because some microwaves are built on assumptions of what sorts of standing waves can form in the microwave and which ones cannot!

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  • $\begingroup$ Why can the mesh on the microwave window safely reflect the microwaves, but a mesh dome could not? $\endgroup$
    – Jaqenn
    Jun 9 '17 at 14:59
  • $\begingroup$ @Jaqenn The answer to that is more complicated because it deals with resonance. The mesh is part of a Faraday cage, which in general actually absorbs microwaves, rather than reflecting them. However, at some frequencies (the resonant frequencies of the chamber), it does indeed "reflect" due to the fact that the electric field at a conductor must be 0. You can think of this like a guitar string, which is attached to the guitar at two points. Any wave which wants to impart a velocity to either of those attachment points is absorbed. However, those strings whose wavelength... $\endgroup$
    – Cort Ammon
    Jun 9 '17 at 15:28
  • $\begingroup$ ... lines up so that the velocity at those points is 0 are "reflected." If you're interested in why, I recommend looking at standing waves. What I've written here just scrapes the surface. However, you'll find that these standing waves also explain why some parts of the microwave are very hot and others are cold. $\endgroup$
    – Cort Ammon
    Jun 9 '17 at 15:30
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metal heats up in a microwave but if your gaps in the links of the mesh are small enough (1mm), it could work. the microwave mesh works because it has supper small gaps between links, which stops the microwaves, which have bigger wavelengths than the gaps. That's like trying to put something big in something that cant hold it, or lets say a trying to put a couch in your car trunk. If your mesh is too thick though, it could cause sparking and burning of the mesh. Even if you find a perfect mesh, it will still heat your food up through absorption of the microwaves, if not, it will heat it up through reflection or letting a couple microwaves through accidentaly, and even if those don't heat your salad up, the surrounding foods and entropy will. If you want to learn more though, the mesh is called a Faraday cage and its really cool. But, answering your question, to be safe, just pay more attention to what your putting on your plate and you should be good.

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