Simple Nature - Light and Matter

m / Discussion question B.Discussion QuestionsA If a light ray has a velocity vector with components c x and c y , whatwill happen when it is reflected from a surface that lies along the y axis?Make sure your answer does not imply a change in the ray’s speed.B Generalizing your reasoning from discussion question A, what willhappen to the velocity components of a light ray that hits a corner, asshown in the figure, and undergoes two reflections?C Three pieces of sheet metal arranged perpendicularly as shown inthe figure form what is known as a radar corner. Let’s assume that theradar corner is large compared to the wavelength of the radar waves, sothat the ray model makes sense. If the radar corner is bathed in radarrays, at least some of them will undergo three reflections. Making a furthergeneralization of your reasoning from the two preceding discussionquestions, what will happen to the three velocity components of such aray? What would the radar corner be useful for?n / Discussion question C.o / The solid lines are physicallypossible paths for light raystraveling from A to B and fromA to C. They obey the principleof least time. The dashed linesdo not obey the principle ofleast time, and are not physicallypossible.12.1.5 ⋆ The principle of least time for reflectionWe had to choose between an unwieldy explanation of reflectionat the atomic level and a simpler geometric description that wasnot as fundamental. There is a third approach to describing theinteraction of light and matter which is very deep and beautiful.Emphasized by the twentieth-century physicist Richard Feynman,it is called the principle of least time, or Fermat’s principle.Let’s start with the motion of light that is not interacting withmatter at all. In a vacuum, a light ray moves in a straight line. Thiscan be rephrased as follows: of all the conceivable paths light couldfollow from P to Q, the only one that is physically possible is thepath that takes the least time.What about reflection? If light is going to go from one point toanother, being reflected on the way, the quickest path is indeed theone with equal angles of incidence and reflection. If the starting andending points are equally far from the reflecting surface, o, it’s nothard to convince yourself that this is true, just based on symmetry.There is also a tricky and simple proof, shown in figure p, for themore general case where the points are at different distances fromthe surface.Not only does the principle of least time work for light in a746 Chapter 12 Optics