国内传统的教材缺乏从波动光学到几何光学的推导过程. 本文通过几何光学近似，提供了一个从波动光学到几何光学的简洁明了的推导.与此同时，本文也顺带得到了光子的能量动量以及粒子数守恒定律与极化适量沿类光测地线的平行移动规律.

There is no obvious derivation of geometric optics from wave optics in those conventional textbooks. This paper presents a concise derivation of geometric optics from wave optics by working with the geometric optics approximation. In passing, the conservation law for both energy momentum tensor and photon number current as well as the parallel transport of the polarized vector along the null geodesic is also obtained.