An ideal converging lens focusses light from a point object onto a point image. Consider only rays that are straight lines except when crossing an air-glass boundary, such as those shown in the figure. Relative to the ray that passes straight through the center of the lens, do the other rays require more time, less time, or the same time to go from \(\mathrm{O}\) to I? That is, in terms of Fermat's Principle, is the central path a local minimum, maximum, or a stationary path that is neither a minimum nor a maximum?