A Faraday cage blocks long wavelength EM waves but does not block short wavelength EM waves.
Shielding of electromagnetic waves by a screen enclosure depends on the wavelength of the EM waves and the size of the holes.
NCEA & Science Curriculum
Set a radio tuned to an AM station on the base of the enclosure. Place the top over the radio and it stops playing. Repeat with an FM station which is stopped or at least dramatically reduced in strength. Both of these signals have wavelengths that are very large compared to the openings in the screen (metres for the FM signal, hundreds of metres for the AM signal). On the other hand, a cell phone signal (tenths of metres) passes through the screen. Have a student place her/his cell phone on the base and cover. Have another student call the cell phone and it will ring!
Individual teachers are responsible for safety in their own classes. Even familiar demonstrations should be practised and safety-checked by individual teachers before they are used in a classroom.
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