FOLLOW US ON TWITTER
SHARE THIS PAGE ON FACEBOOK, TWITTER, WHATSAPP ... USING THE BUTTONS ON THE LEFT


YOUR PARTICIPATION FOR THE GROWTH OF PHYSICS REFERENCE BLOG

Sunday, October 27, 2019

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.

No comments:

Post a Comment

If it's a past exam question, do not include links to the paper. Only the reference.
Comments will only be published after moderation

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