Group velocity and phase velocity

When I recently wrote about observing waves in a different way, I talked about light being focussed by the waves on the sea floor.

In the other post, I focussed on how looking at the light and dark pattern on the sea floor makes waves visible that are otherwise hard to see when just looking at the surface of the water:

But it also makes something else easier to notice, and that is how phase velocity and group velocity are really different. We know they should be, but on a choppy water surface it is really difficult to keep track of individual waves as they wobble through each other. But looking at the light and dark pattern on the sea floor, it actually becomes easier to observe. See those brighter areas and darker areas? Wave groups!

And in the movie below, you’ll see how they eat up waves that run into them, and how other wave crests comeĀ out in the front as they are overtaking the group. Cool! :-)

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