  Considering the Cornell potential $V(r)=-\frac{4\alpha_s}{3r}+br+c$, we have
revisited the Dalgarno's method of perturbation by incorporating two scales
$r^{short}$ and $r^{long}$ as integration limit so that the perturbative
procedure can be improved in a potential model. With the improved version of
the wave function the ground state masses of the heavy-light mesons $ D, D_s,
B, B_s$ and $B_c $ are computed. The slopes and curvatures of the form factors
of semi-leptonic decays of heavy-light mesons in both HQET limit and finite
mass limit are calculated and compared with the available data.
