  Observations of metallic doublet emission lines, particularly Mg II 2796,
2803, provide crucial information for understanding galaxies and their
circumgalactic medium. This study explores the effects of resonant scattering
on the Mg II doublet lines and the stellar continuum in spherical and
cylindrical geometries. Our findings show that under certain circumstances,
resonance scattering can cause an increase in the doublet flux ratio and the
escaping flux of the lines beyond what are expected in optically thin spherical
media. As expected, the doublet ratio is consistently lower than the intrinsic
ratio when the scattering medium is spherically symmetric and dusty. However,
if the scattering medium has a disk shape, such as face-on disk galaxies, and
is viewed face-on, the doublet ratio is predicted to be higher than two. These
results may provide a valuable insight regarding the complexity of the shape
and orientation of distant, spatially-unresolved galaxies. The importance of
the continuum-pumped emission lines and expanding media is discussed to
understand various observational aspects, including doublet flux ratios, which
can be lower than 1.5 or higher than two, as well as symmetric or asymmetric
line profiles. It is also discussed that the diffuse warm neutral medium would
be an essential source of Mg II emission lines.
