Recent metasurface developments have led to their consideration for extending the angular scan range of phased arrays. This paper considers the use of Huygens' metasurfaces as lenses to accomplish the task. Ray optics is used to uncover how a single lens can extend the scan range. It is shown that a single scan extending lens leads to a broadside directivity degradation of the original beam. This directivity degradation is quantitatively characterized as a function of the desired angular scan-range expansion. A method of simulating phase boundaries is presented and used to verify the theoretical claims. A Huygens' metasurface lens design is presented and simulated to further validate the theoretical predictions and show that such lenses are physically realizable.