Q#26 (Past Exam Paper – November 2017 Paper 13 Q30)
The diagrams show four pairs of waves. In each case the displacement y measured at a fixed point is plotted against time t.
Which pair of waves is not coherent?
Solution:
Answer: C.
Waves are said to be coherent if they have a constant phase difference. That is, the phase difference does not change with time.
Note that a constant phase difference does not mean that the phase difference is zero. There may be a phase difference but this value needs to be constant – it should not be changing.
However 2 waves have a constant phase differences implies that the periods / frequencies need to be the same. If the 2 waves have different periods, the phase difference would keep on changing with time.
So for a pair of waves to be coherent they should have a constant phase difference. This implies that they should have the same period.
Option C is incorrect as the waves have different periods.
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