Fermat's Principle $-$ The "Principle of Least Time" asserts that light will always follow a path between two points that minimizes the time of traversal between those points.Note that for straight rays, the time of traversal for light in medium with index n isI $==$ n$,$ where l is the length of the ray.$($a$)$ Use this principle to prove the Law of Reflection $(0$; $=0,)$ with the geometry shown.