Fermat’s principle states that light follows the path of least time. In textbooks, a specific formulation of Fermat’s principle is about the optical path between two points, A and B: How can a ray of light, emitted from point A, reach point B? Suppose that there is a plane wave in free space, with the line AB not parallel to the wave vector. In such a case, the actual light ray never goes from A to B because the actual light ray must be perpendicular to the equiphase planes. From this, a question comes to me: Is the Fermat’s principle appicable to a plane wave?