  We present a semi-classical theory for light deflection by a coherent
$\Lambda$-type three-level atomic medium in an inhomogeneous magnetic field or
an inhomogeneous control laser. When the atomic energy levels (or the Rabi
coupling by the control laser) are position-dependent due to the Zeeman effect
by the inhomogeneous magnetic field (or the inhomogeneity of the control field
profile), the spatial dependence of the refraction index of the atomic medium
will result in an observable deflection of slow signal light when the
electromagnetically induced transparency happens to avoid medium absorption.
Our theoretical approach based on Fermat's principle in geometrical optics not
only provides a consistent explanation for the most recent experiment in a
straightforward way, but also predicts the new effects for the slow signal
light deflection by the atomic media in an inhomogeneous off-resonant control
laser field.
