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Waves are vibratory disturbances travelling in a medium. If you understand that sentence, you have understood wave motion. Shown above are vibrating layers of air in a horizontal tube. The layer at the extreme left is the one disturbed earliest. You could imagine a vibrating piston at the left end causing these disturbances. So the layer at the left extreme is always ahead in phase of the layers to its right. The time lag between the moments when different layers begin oscillating causes the layers to come closer to each other at some moments and move farther at some other moments. To get a feel of this, imagine what would have happened if all the layers vibrated in phase. You would always have layers maintaining the same distance between themselves and you would not have any wave disturbance traveling in the medium. The key to understanding of wave motion is this simple idea that phase is different for the different vibrating particles because they start vibrating at different times. The ones farther from the source osciallating later than the ones closer to the source.