1 Traveling Wave

From here you may load this preset , see the text of the next , or return to the preset list.

Displayed is a 60 atom segment of an infinite chain of atoms. An initial set of atom displacements is shown, varying as A cos(qna) where q is the wave vector. The continuous curve A cos(qx) is also shown. A click on run sets the system in motion as appropriate to a traveling wave. The graph shows the displacement from equilibrium of the central atom versus time. The elapsed time from the beginning of the run is in the upper left corner. It may be used to determine the phase velocity of the wave.

Select triple from the display menu and run the program for a few cycles. The graph shows the motion of the center atom (blue) and the next two atoms to its right (in green and red). (To see features of the graph in more detail, see the Graphs page for zooming instructions.)

The discreteness of the atomic chain can lead to some confusion of interpretation. Double click in the wave vector window, type "0.9" and hit the enter key. The eye picks up two interleaved patterns of long wavelength, but the continuous curve is of very short wavelength. Move the speed slider to 1 and run. Is this wave moving to the right or to the left?

Change the wave vector to 0.2. Now type 2.2 into the wave vector box and watch the atoms carefully as you hit the enter key. Many choices of wave vector yield the same pattern of atomic displacements.