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Why Slow Light from a Wavelength-Dependent Wave Speed?

 

To understand how a wave speed that depends strongly on wavelength can
lead to slow light signals, Welch suggests imagining a pair of overlaid
combs. If the combs travel at slightly different speeds, the group
velocity of the patterns they create can be dramatically different
(faster in this case) than the speed of the combs themselves. The
wave mathematics are also similar for two sound waves that have
slightly different frequencies--the combination generates a much slower
wave with the so-called "beat" frequency.

 

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