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From a user who "doesn't know" anything about Maxwell's equations. How to explain simply the behavior of a transparent glass of a microwave oven? Lets the light pass ... but blocks the "dangerous" "radiation" emitted by a powerful generator of electromagnetic waves (~ 1 kW). How's it going ? How to explain the manufacture of this glass slab?

Thanks to all contributors and comments. Below my answer at the question. (not complete ...) I don't know if it is usefull, but I hope so.

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2 Answers 2

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It isn't the glass in the microwave oven door that blocks the microwave radiation.

The microwave radiation is blocked by the metal plate with holes behind the glass.

The inside of a microwave oven is a Faraday cage. It blocks (most) electromagnetic radiation.

The small, closely spaced holes in the metal allow light through but still block the microwave radiation. This works because electromagnetic waves require as much "space" to pass through as the wave itself is long.

The microwaves used to heat food have a wavelength a couple of centimeters long. They cannot pass through the millimeter sized openings in the metal grid.

Light has a wavelength that is in the range of thousandths of a millimeter. Light waves can easily pass through the millimeter sized openings in the metal grid.

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  • \$\begingroup\$ ok. Thank you. With "How to explain the manufacture of this glass slab?", I was thinking also for that "grid". \$\endgroup\$
    – Antonio51
    Commented Jul 1, 2021 at 9:09
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    \$\begingroup\$ JRE: a microwave oven is not a Faraday cage, it's a cavity for a specific wavelength only. Test: (0. unplug the microwave to mitigate any chance of tragic accidents), 1. put phone in microwave 2. call phone \$\endgroup\$
    – Marcus Müller
    Commented Jul 1, 2021 at 9:39
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    \$\begingroup\$ @ChristianidisVasileios: Nope. Wavelength. \$\endgroup\$
    – JRE
    Commented Jul 1, 2021 at 9:39
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    \$\begingroup\$ @MarcusMüller: A microwave oven is a Faraday cage. The cooking "cavity" itself might be sized to prevent (or enhance) standing waves in an attempt to prevent hot spots. The attenuation isn't perfect, of course. If I put my phone in my microwave, I cannot call my phone because the signal from the cell phone tower is fairly weak where I am. \$\endgroup\$
    – JRE
    Commented Jul 1, 2021 at 9:42
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    \$\begingroup\$ @JRE fair point, and I wish you better reception (unless you do sensitive measurements, in which case I hope the phone tower crumbles and falls ;) ); but I assure you that assuming the magnetron is tuned to 2.45 GHz, a 900 MHz, 1.8 GHz or 3 GHz cellular signal isn't that strongly attenuated. Do I whip up an SDR+single board computer now to log signal strengths while I put it inside? Hm, maybe after work. \$\endgroup\$
    – Marcus Müller
    Commented Jul 1, 2021 at 9:46
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The glass or plastic has nothing to do with microwave blocking. It's the metal mesh behind the glass or plastic that keeps the microwaves inside the oven.

Light can come through those holes in the metal but the holes are so small that microwaves can't fit through them, as the wavelength is much larger than wavelength of light which can fit through.

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  • \$\begingroup\$ Ok. But are they same type "waves" as others in a coaxial cable ? And how simply can we know which pass through or not ? Ok, It can be another question. \$\endgroup\$
    – Antonio51
    Commented Jul 1, 2021 at 9:15
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    \$\begingroup\$ @Antonio51 there's no different types of waves, it's all EM waves. Maxwell's equations describe the magnetic and the electric field and their relation to each other and time. Visible light, X-ray, infrared, terahertz radiation, radio waves: all the same. \$\endgroup\$
    – Marcus Müller
    Commented Jul 1, 2021 at 9:40
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    \$\begingroup\$ That good friend is right, but your good friend assumes that you can tell the difference, which requires that the first sentence of your question is wrong. \$\endgroup\$
    – Marcus Müller
    Commented Jul 1, 2021 at 10:37
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    \$\begingroup\$ @Antonio51 well, then find a very polite way tell him to leave you alone with half-knowledge; it's very hard to know what your friend knows when he doesn't know basics but has heard of some special concepts arising from that :) The concept of TM,TE and TEM is something that only makes sense if you understand the relative directions of E and H field variations and propagation, and none of that exists without at least a basic understanding of Maxwell ! \$\endgroup\$
    – Marcus Müller
    Commented Jul 1, 2021 at 10:50
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    \$\begingroup\$ well, the fact that TE and TM and TEM are different in the direction of fields has absolutely no influence on the problem at hand, so I wouldn't know why to explain it, which is great because I don't think you can explain the direction of propagation from the fields without essentially explaining Maxwell. \$\endgroup\$
    – Marcus Müller
    Commented Jul 1, 2021 at 11:26

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