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I was looking at my microwave the other day and I thought that the metal mesh screen might be acting like a set of perpendicularly oriented polarizers. Other explanations I've heard on how this screen blocks out radiation seem to hinge on the wavelength of microwaves being large enough that the spacing of the mesh screen effectively looks like a solid screen to incoming microwaves. Is it accurate to think of the mesh screen like a set of polarizers? If not then could the metal mesh screen be replaced by a mesh of any arbitrary material and still block out radiation?

Phaidros
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  • Would you consider a brick wall as a set of polarizers for visible light? – probably_someone Jun 27 '17 at 23:42
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    No. So then does that mean that the mesh screen doesn't necessarily need to be metal? Would a wooden mesh work just as well, or a glass mesh? I thought they might be polarizers because they looked like polarizers, but is that just coincedental? – Phaidros Jun 27 '17 at 23:55
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    Metal is used because it reflects radiation. Wood or glass might absorb a tiny bit and reflect even less, but most would be transmitted (unless the wood is wet, then the water inside would absorb it). – probably_someone Jun 28 '17 at 00:16
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    So then the metal mesh shields from radiation partly because it is reflective and partly because grid spacing is comparatively small for microwaves? – Phaidros Jun 28 '17 at 00:22
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    Precisely. A metal sheet without holes would work just as well, but then you wouldn't be able to see inside. – probably_someone Jun 28 '17 at 00:24
  • Thinking about it again, I wouldn't consider a brick wall to act like a set of polarizers for visible light, but I think a metal wall functions kind of like a set of perpendicularly oriented polarizers as the spacing between bars on the polarizers approaches zero. Is this inaccurate? – Phaidros Jun 28 '17 at 00:26
  • In what situation would that viewpoint be useful? – probably_someone Jun 28 '17 at 00:45
  • I don't know. Does it have to be useful? I guess my problem is that I'm not convinced that we can't treat the mesh like a set of polarizers. It seems to me that they look and behave like polarizers. Is there something wrong with that view point? Are the two descriptions of the behaviour of the mesh screen contradictory? – Phaidros Jun 28 '17 at 01:18
  • What do you mean when you say they behave like polarizers? – probably_someone Jun 28 '17 at 01:23
  • My understanding is that linear polarizers are composed of a grating of conducting material, with each particular bar of metal thin enough that oscillations stimulated by light along the width of the bar are virtually nonexistent. Oscillations of electrons in the conducting metal along the length of the bar re-radiate light that when summed up cancel out (idk exactly how but my book said that the math works out) so that only light with polarization perpendicular to the grating is transmitted. My thinking is that the mesh screen looks like this and should behave like a set of linear polarizers. – Phaidros Jun 28 '17 at 01:35
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    In order for a polarizer to work, the length of the grating (not the spacing between the slats, but the length of said slats) should be greater than the wavelength of the waves. The wavelength of microwaves in a microwave oven is roughly 12 cm. The openings in the mesh of a microwave are typically less than 1 cm on a side. So it is literally not possible for the mesh to act as a polarizer. – probably_someone Jun 28 '17 at 01:44
  • So then if the mesh screen were larger. with the slats being the same width with the same spacing between them, it would act like a linear polarizer? – Phaidros Jun 28 '17 at 01:56
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    Well, you'd have to change the openings from squares to very thin rectangles, but yes, that could work. The downside, of course, is that now you're cooking the rest of your kitchen as well. – probably_someone Jun 28 '17 at 02:02
  • Why would the openings need to be rectangles? – Phaidros Jun 28 '17 at 02:12
  • They don't, I just chose a simple shape. Whatever shape you choose, though, it must have a very high aspect ratio (i.e. it must be thin). – probably_someone Jun 28 '17 at 02:14
  • I'm not sure I follow. If we made these large polarizers and they both have the same spacing between the slats, and we then stacked them perpendicular to each other, wouldn't the openings look like squares? – Phaidros Jun 28 '17 at 02:19
  • Also, how would I arrive at the result that the length of the slats must be larger than the wavelength of incident light? – Phaidros Jun 28 '17 at 02:20
  • If you stack two perpendicular polarizers, then you no longer have a polarizer - you have a wall. – probably_someone Jun 28 '17 at 02:28
  • Read this for why: https://physics.stackexchange.com/questions/141562/diffraction-by-small-holes – probably_someone Jun 28 '17 at 02:30
  • Ok, thanks for the help and resources. Sorry to take up so much of your time. – Phaidros Jun 28 '17 at 02:54

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