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.
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