# Why must a Faraday cage be hollow?

A metal piece has no E field inside too. Why must the microwave use a metal mesh with hollow spaces to block radiation? They can simply use a metal piece just like the interior part of the box.

atyy

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For economic reasons?

atyy
russ_watters
Mentor
So you can see through the window.

mrspeedybob, atyy, nsaspook and 1 other person
davenn
Gold Member
Why must the microwave use a metal mesh with hollow spaces to block radiation? They can simply use a metal piece just like the interior part of the box
So you can see through the window.

exactly

atyy
Baluncore
Why must the microwave use a metal mesh with hollow spaces to block radiation?
Note that it is not a mesh that is used but it is a sheet of perforated metal. That is easier to clean and more conductive. If the dimensions of a hole are all less than about one tenth of a wavelength then microwave radiation cannot radiate through the hole. You can see inside through the perforations because light has a much shorter wavelength than microwaves. The outside of the perforated sheet is painted black to make it easier for you to see inside without problems from external lighting.

The 2.45GHz microwave radiation used has a wavelength of 12 cm so you might ask why they do not use 12 mm holes? The answer is that, from a distance, you can see through many small holes better than through a few large ones.

atyy
Note that it is not a mesh that is used but it is a sheet of perforated metal. That is easier to clean and more conductive. If the dimensions of a hole are all less than about one tenth of a wavelength then microwave radiation cannot radiate through the hole. You can see inside through the perforations because light has a much shorter wavelength than microwaves. The outside of the perforated sheet is painted black to make it easier for you to see inside without problems from external lighting.

The 2.45GHz microwave radiation used has a wavelength of 12 cm so you might ask why they do not use 12 mm holes? The answer is that, from a distance, you can see through many small holes better than through a few large ones.
Thin metals become very hot when they are put into the microwave oven because the ke of electrons increases while it reflects microwave and may produce spark. Why won't the perforated metals become very hot and produce spark?