It is shown that the energy levels are shifted when a hydrogen atom is
placed in a finite space formed with two parallel metallic plates. Th
e 2S1/2 and 2P1/2 states of the hydrogen are shifted by -0.895 MHz and
-0.481 MHz, respectively, for 1 mum separation between two plates. Th
e lifetime of the hydrogen 2P1/2 state is also shortened by 23%. We in
vestigate the effects of the opaqueness of the metallic plates.