Beats

Applet shows two waves travelling to the right. Imagine they are travelling along the same line, but they are shown to be travelling along parallel paths as a matter of convenience. When the two waves are received by a particle, the resultant vibration the particle would execute is shown in yellow. You can change the frequency difference beween the waves and the phase difference. Observe the motion of the white dots. It is the particular particle you could focus on. Except in case when the frequency difference is zero, you would see the phase differnece between the oscillations varying with time. Since the resultant amplitude depends on the phase difference between the two oscillations you see maximum amplitude when they are in phase and minimum when they are out of phase. They keep getting in and out of phase resulting in maxima's and minimas at different times at the same place. We call this interference in time. The number of times the particle gets in and out of phase would depend on the frequency difference between the waves. If you increase the frequency difference, you would see larger number of maximas and minimas.

Do also observe the case of zero frequency difference. You would see that the waves have the same phase difference at all times giving rise to resultant oscillations of constant amplitude- either zero or twice the amplitude of one of the waves depending on whether phase difference is PI or zero .