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In order to see the difference between group
velocity and phase velocity, let's make a wave group.
To make this experiment work, we need to make
"enough waves to form a group". So we will
increase the number of wave making rods to four, and
make four wavelengths of wave at once.
If you create the waves in the upper right corner,
one of the wave groups spread from the wave source
toward the bottom left. Of course energy is also
being transmitted in that direction. Inside the wave
group, the waves' phase (their bow and trough
pattern) moves from bottom right to top left. This
pattern does not leave the wave group's area, so
waves form one after the other at the bottom right of
the group and disappear one after the other at the
top left.
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