[jerf]

"(and it's highly frequency dependent on top of that.)"

Well, sure. Can people inside the cage see outside? (Or a hypothetical person for a small cage.) If so, then clearly, not all frequencies are being blocked. A lot of "Faraday cages" are explicitly designed for radio and deliberately let other frequencies, particularly the visual range, through.

In fact we all have direct experience with that. Our microwaves use a Faraday cage to keep them in. But we can still see through the mesh, and you can tell that the inside can see out because outside light can go in and bounce back out. (That is, while there's probably a light in your microwave, it's obviously not the sole source of light.) Blocks microwaves well, but visible light goes right through the holes.

[wat10000]

They let out enough to interfere with radios operating around 2.4GHz. They'll attenuate the stuff, quite strongly if built well (the only reason interference is a problem is because the oven is 3+ orders of magnitude more powerful than a typical 2.4GHz radio), but it's not a total block.

[giantg2]

Anyone interested can test this with an RF bug finder, even the homebuilt ones that just increase the intensity of an LED when near a source will work to demonstrate the leaks.

[grogenaut]

"""General rule of thumb is that the opening in a Faraday cage should be smaller than 1/10th of the wavelength that should be blocked. For example, in order to block EM fields with frequencies of 10 GHz and lower, the hole size of the Faraday cage should be smaller than 3 mm."""

wavelength of red in inches: 2.46063e-5 wavelength of 2.4ghz in inches: 4.25 wavelength of xrays in inches: 7.87402e-7 (upper end)

You could easily see through a 4.25 mesh, that's almost chain link

you could not see through a 2.4e-5 mesh, that's call fabric, unless you can see through clothes in which case I'm not going near you. xrays can see through that :)