Group Velocity
Purpose
Superimposing two transparency sheets with different line spacings creates a beat pattern (moire effect). When one sheet slides across the other, the beat pattern moves much faster than the sheet itself, demonstrating the distinction between phase and group velocity.
Figure 1:
90% and 100% line density transparencies overlaid, showing a subtle moire beat pattern.
Figure 2:
80% and 100% line density pair. The larger frequency difference produces wider, more pronounced beat bands.
Figure 3:
95% and 100% line density pair with lines parallel, producing clear vertical beat bands.
Figure 4:
95% and 100% pair with one sheet slightly rotated, producing dramatic diagonal moire bands. Sliding one sheet creates fast-moving beat patterns.
Demo Apparatus
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