So I'm a little confused. My question is: If a conductor is placed in an external electric field, do the field lines penetrate the conductor? My original thought was yes they do and then the induced field inside the conductor cancels out the external field so that the net field inside the conductor is zero. But, in griffiths (3rd edition p. 99), he introduces the problem of a conductor with a cavity inside it. The cavity encloses a single point charge q. Griffiths says "But in a remarkable way the cavity and its contents are electrically isolated from the outside world by the surrounding conductor. No external fields penetrate the conductor; they are canceled at the outer surface by the induced charge there." So is it that in this particular problem an external field wouldn't penetrate the conductor or the cavity? Thanks.