  The Ryu-Takayanagi formula provides the entanglement entropy of quantum field
theory as an area of the minimal surface (Ryu-Takayangi surface) in a
corresponding gravity theory. There are some attempts to understand the formula
as a flow rather than as a surface. In this paper, we propose that null rays
emitted from the AdS boundary can be regarded as such a flow. In particular, we
show that in spherical symmetric static spacetimes with a negative cosmological
constant, wave fronts of null geodesics from a point on the AdS boundary become
extremal surfaces and therefore they can be regarded as the Ryu-Takayanagi
surfaces. In addition, based on the viewpoint of flow, we propose a wave
optical formula to calculate the holographic entanglement entropy.
