Purpose:Demonstrate the relationship between the wave velocity and the group velocity
Description:A set of two 8"x11" transparencies with closely-spaced parallel lines, are slowly slid across each other. One has a spacing about ten percent smaller than the other, so the difference in spacing creates a Moire interference pattern. Wave groups move rapidly across the picture while the wave velocity (motion of the transparency) is much smaller. Note that the scale of the photo at the bottom, containing the superposition of the two transparencies, is greater than that of the top photograph containing the component transparencies.