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Interference of light and constant phase difference

I don’t understand why waves need a constant phase difference in order to interfere, could anyone help please?
'Interfere' in this context usually means 'combine or add in such a way as to produce a stable and recognizable pattern of light and dark fringes.'

Any 2 waves can meet and combine, and the resultant displacement will be the (vector) sum of the amplitudes at that point. That isn't normally called 'interference' unless it causes the effect I described above. And in general, when random waves meet and combine, there is no interference pattern visible.

Now we know that if 2 waves meet such that a crest meets a trough, the 'vector' sum can produce a zero resultant amplitude. We say the 2 waves are out of phase by 180 degrees, or half a wavelength. They have a 'phase difference' of 180 degrees.
If this zero resultant amplitude (a dark fringe) is to remain visible, then the 2 waves must remain 180 degrees out of phase.
This happens when we have so called coherent waves, waves which keep a constant phase difference between them.
They will, of course, have the same frequency, as this is a condition for them to be able to keep their same phase difference.
(edited 4 years ago)
Original post by Stonebridge
'Interfere' in this context usually means 'combine or add in such a way as to produce a stable and recognizable pattern of light and dark fringes.'
....


Welcome back to TSR, it has been a long time since you post it the last time.

I think in physics, it is usually written as "bright and dark fringes" rather than "light and dark fringe".
Original post by Eimmanuel
Welcome back to TSR, it has been a long time since you post it the last time.

I think in physics, it is usually written as "bright and dark fringes" rather than "light and dark fringe".

Thank you for the welcome. I was away for about 5 years.
Yes indeed, usually called 'bright' fringes. Fortunately, the use of the term 'light' here does not detract from the explanation. Well spotted. :smile:
Original post by Stonebridge
Thank you for the welcome. I was away for about 5 years.
Yes indeed, usually called 'bright' fringes. Fortunately, the use of the term 'light' here does not detract from the explanation. Well spotted. :smile:


Could not upvote for the reason that I believe you know: PRSOM. :smile:

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