| The motion of a narrow banded wave group can be regarded as a simusoidal
wavetrain (blue dotted line) with central frequency
and wave number, while the envelope (red solid line)
is slowly varying in space and time. The speed of the envelope is called
group velocity. The periodic variation of the
envelope in space forms many wave packets. The group velocity is just the
speed of these wave packets. From the dynamics view, group velocity has
a physical significance-- the rate of energy transport, which makes it more
important than the phase velocity. |
