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Animation 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
between 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 ssee the phase
difference 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 minima 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 maxima and minima.
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 π or zero .
Double clicking shows up two vertical lines. You can drag
them to help you to count the waves between successive crest , crest and crest ,
trough coincidences.
Double clicking a second time hides the lines.