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A periodic function is an event that occurs repeatedly in a very regular manner. Suppose that you stand on the beach near a lighthouse. As you watch, the lighthouse's beam will sweep across the landscape in a very regular pattern. You might see the beam pointing directly at you every second, or every two seconds, or at some other regular interval. The

regularity of a lighthouse beam is one of the ways in which ships at sea recognize the lighthouse and know where they are.

The graphical representation of a periodic function has a very characteristic shape. Figure 1 shows the most common of these shapes. In this graph, the vertical (y) axis shows the amplitude (magnitude) while the horizontal (x) axis shows time. As you go from left to right along the graph, it is as if you were moving forward in time. The origin (the point at which the two axes cross) represents zero time, and each additional second or minute or hour is...