The applet shows two waves travelling to the right. Imagine that they are travelling along the same line. When the two waves reach a particle, the particle vibrates as shown by the signal in yellow. The white dots represent isolated particles. Except in the case when the frequency difference is zero, the phase difference between the oscillations varies with time. Since the resultant amplitude depends on the phase difference between the two oscillations, the amplitude is maximum when the waves are in phase and minimum when they are out of phase. The two waves keep getting in and out of phase resulting in their combination displaying maxima and minima at different times/places. This is called interference in time/space. The number of times the particle gets in and out of phase depends on the frequency difference between the interfering waves. If you increase the frequency difference, you will see a larger number of maxima and minima.

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