In this example, the high-frequency wave has a higher phase velocity than the low-frequency wave. This is called anomalous dispersion. As a result, the group velocity is much greater than the phase velocity of the individual waves; that is, the packets move a lot faster than the single-frequency waves.

In some cases, the group velocity of light wave packets can be faster than the speed of light. This is often mistaken for faster-than-light travel in news reports. Click the Restart button to see why this cannot be used for faster-than-light communication; the signal velocity is not faster than the speed of light.

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