  The inverse time ordered Green's function is defined in a covariant
three-dimensional formalism using the diagram approach.The equation of motion
of a two-quark composite system in an external electromagnetic field up to the
second order in the field strength is obtained. The polarizability of the
$\pi$-meson is calculated using this formalism. The mean square radius of the
two-quark system is calculated from the wave functions of the system. In
addition, radiative corrections are calculated for both the polarizability and
the mean square radius.
