The Law of Reflection

Tennis players can generally predict which direction a ball will go as it bounces off the court.  We can also predict the direction of a reflected light beam.  Light reflects off a smooth surface in the same manner that a ball reflects off a smooth tennis court.  This behavior is described by the law of reflection.  The law of reflection simply states that the angle of reflection (as measured to the normal) always equals the angle of incidence (as measured to the normal):

θi = θr.
This relationship is illustrated below.
Animation illustrating law of reflection In the figure to the left, a laser beam traveling through an optical fiber (shown in incredibly slow motion) reflects off the lower edge of the fiber. The normal to that edge is indicated by the dashed line.  As the beam approaches the edge of the fiber, it makes an angle of incidence θi with respect to the normal.
  A. The reflected beam travels off at an angle of reflection θr with respect to the normal.
  B. The law of reflection says that this angle of reflection will always equal the angle of incidence θi.
Sample Problems
Practice Exercises
  At-Home Assignments
Why doesn't a shortstop always correctly predict how the ground ball will bounce? 
Does he not know the law of reflection?  Is the law not always valid? 
Go to the next page to learn when the law of reflection appears not to hold!

Copyright © 1999-2004 Doris Jeanne Wagner and Rensselaer Polytechnic Institute.  All Rights Reserved.