Containers of metals like copper, steel, or aluminum work well as a Faraday Cage to shield electromagnetic waves of a wide range. But if we go to the end of very high energy radiation (like gamma rays), these containers used cannot prevent rays from penetrating inside.why?
The answer can be understood with a very simple classical model of a metal being a neutral plasma of basically free electrons and immobile positively charged atomic ions. At low (radio and microwave) frequencies the electrons follow the oscillating electric field of the em waves and dissipate the energy by collisions with phonons thus attenuating the penetration of the waves. At higher frequencies the electrons cannot follow the electric field anymore and the collisions become ineffective. Above a critical frequency called plasma frequency, which depends on the electron concentration (around 10^23 cm-3 in metals), the metals become practically transparent to electromagnetic waves. In common metals this happens in the ultraviolett range of the optical spectrum. Therefore, high energy electromagnetic radiation like x- and gamma-rays are not shielded by metal Faraday cages.