#2 - Two modalities of giving the sensation of movement.

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Both ordinary waves and those caused by the variation in water density give the sensation of movement. In the first case, the wave advances, but the movement of the molecules is reduced. In the second case, the movement is almost zero.

In the density waves.

In density waves, movement is only an impression given by the varying density of water. This is my interpretation.

In the case of density waves, in each point of the considered area, density variation processes of the water would be underway.

Each point, at a given moment, is in a phase of the process, to which phase it passes immediately after the next point.

The contemporary vision of all points of the entire area considered gives the impression of a moving wave. What you see, at a given moment, is at what stage of the process the various points of the picture are located. Dynamic process, which causes the impression of movement.

The volumes occupied by the individual molecules vary.