  The Dirac operator, acting in three dimensions, is considered. Assuming that
a large mass $m>0$ lies outside a smooth and bounded open set
$\Omega\subset\R^3$, it is proved that its spectrum is approximated by the one
of the Dirac operator on $\Omega$ with the MIT bag boundary condition. The
approximation, which is developed up to and error of order $o(1/\sqrt m)$, is
carried out by introducing tubular coordinates in a neighborhood of
$\partial\Omega$ and analyzing the corresponding one dimensional optimization
problems in the normal direction.
