We consider optical pumping in the multilevel sodium D1 manifold by se
veral coherent light fields. Coherence between atomic states is taken
into account by solving the complete density matrix equations. The man
ifestation of population trapping and the variation of transmitted lig
ht intensity are predicted numerically and verified experimentally for
the sodium D1 line. The influence of relaxation and Doppler broadenin
g is elucidated. The interaction of a strong and a weak field is shown
to create electromagnetically induced transparency.