Abstract: 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. 

Key words: source free Maxwell equation, geometric optics approximation, null geodesic, conservation law, polarized vector