The diagrams show four pairs of waves. In each case the displacement y measured at a fixed point is plotted against time t.

Question 26

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?

Reference: Past Exam Paper – November 2017 Paper 13 Q30

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.