# What restricts two separte sources of light to act coherent?

• ovais
In summary, coherence is a property of a single source that can be improved with lasers, but it is not possible to create a perfectly coherent source.
Drakkith said:
The phase of the wave changes over time at all locations, not just x=0.
OK so this thing that is change of phase(at x=0 too) with time will also happen with point Q. Isn't?

If it phase of wave at P(at x=0 also) changes and at Q also changes. What will you say if the phases of waves from P and Q at any time say t(or t=0) bear a common value? (source is, as remarked monochromatic).I hope(at any time t) phase of P(at x=0)and phase of wave(at the same time t), different values.

If such is the case?

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ovais said:
OK so this thing that is change of phase(at x=0 too) with time will also happen with point Q. Isn't?

It occurs everywhere. It's one of the defining properties of a wave.

ovais said:
If it phase of wave at P(at x=0 also) changes and at Q also changes. Can what will you say if the phases of waves from P and Q at any time say t(or t=0)? I hope(at any time t) phase of P(at x=0)and phase of wave(at the same time t), different values.

I have no idea what you're asking here.

Ok

Drakkith said:
I have no idea what you're asking here.

I am asking if I focus simultaneously at two points P and Q of my illuminated source(monochromatic). Say I note the phase of wave from P( at x=0,t=0) as alpha. Will the phase of wave from Q(at x=0,t=0) which I noted will come alpha or not?

ovais said:
Say I note the phase of wave from P( at x=0,t=0) as alpha. Will the phase of wave from Q(at x=0,t=0) which I noted will come alpha or not?

I already answered this in post #26. The phase is identical at both points.

Drakkith said:
I already answered this in post #26. The phase is identical at both points.
Oh sorry :-) yes you answered it. Ok so it means that at every point on the source(monochromatic) all the waves(from respective) points will be in (same) phase.

Is their some explanation to support this? I am not saying this because of any doubt. Of course you would be definitely correct. I know I must be wrong in my intituion that the wave from two points is the result of different electron movement( production of light wave is however as complex phenomenon not simple electron/atom movement as told by you), the waves(at any time at =0) should have different phase(as the two waves are the result of excitement of different electrons).

I really feeling helped. My questions have now been answered. The only think I find a problem is to comprehend the fact you are telling that phases of P and Q(at t=0,x=0) will be same though I accept it to be true. It would be of more help if you explain or refer some book that has an explanation of this.
Regards

I don't know if I can find a reference that specifically says that the phase at P and Q will be the same. It's kind of a given under the conditions in our imaginary setup.

@ovais After all these posts, I am still not sure want it is that you are after. If it's just how to interpret the way that waves are described mathematically then that is straightforward. But you seem to be trying to relate it to practical situations. The way into a subject like this is not to try to tackle the clever practical stuff but to get the fundamentals right first.
How is the term "coherent' so relevant for you, here? Have you grasped the significance of the term "coherent" and what it means in practice? It doesn't just refer to time synchronism of two sources where the signals are generated but on how they are synchronised at a point, elsewhere in space.
At a given point in space, the phase relationship between the received signals will not be the same for all frequencies - because of the path difference will change. So I can't see what you are getting at and why it is concerning you so much.

Drakkith said:
I don't know if I can find a reference that specifically says that the phase at P and Q will be the same. It's kind of a given under the conditions in our imaginary setup.
You seem to be having a similar problem to mine in understanding what ovais actually wants. The phase relationship between two 'coherent' sources will not be the same everywhere in space - that's why we get interference patterns. Moreover, when he talks of varying the frequencies of the two sources, he is building in a situation where the phase relationships actually change in time and the interference pattern will be moving about as the frequency varies. It's just building in complexity on top of complexity when I think what he needs is to understand what Coherence is all about.

Drakkith
Here I attach the picture of the content of my textbook saying Young Locked the phases. I created this thread for a search that if he had to lock the phases(which would be changing) due to the problem with his source that sodium source undergo abrupt phase changes or due to problem of frequency. Initially it was told to me here that one can't change phase without change in frequency and that phase and frequency can't be changed independently. Later today Drakkith talk more openly saying the phase of wave(at x=0,t=0) varies no matter source is monochromatic or not. Means even if frequency do not change phase can change. I was after this from many days and today only he accepted it.

My last question that I posted today that if the phases of waves from P(at t=0,x=0) and Q(at t=0,x=0) has a physical importance to answer, what actually was Young controlling (to get a constant phase); the changing frequency(with time) of wave from a single point of main source or that he could not use a single source(monochromatic) as coherent source because the different points on the source(at t=0,x=0)have different phases(the reason which I felt). According to me if at the different points say P and Q of the (monochromatic) source he could get waves with same phase(at t=0,x=0) he would let the interference happen with only one source since such a hypothesis that phases of P and Q of, (monochromatic) source will be same(at t=0,x=0) will about to talk the single source as being coherent source. In that case he won't need two slits. If at any time the waves from different points of sources emerges with common same phase.

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ovais said:
Here I attach the picture of the content of my textbook saying Young Locked the phases. I created this thread for a search that if he had to lock the phases(which would be changing) due to the problem with his source that sodium source undergo abrupt phase changes or due to problem of frequency. Initially it was told to me here that one can't change phase without change in frequency and that phase and frequency can't be changed independently. Later today Drakkith talk more openly saying the phase of wave(at x=0,t=0) varies no matter source is monochromatic or not. Means even if frequency do not change phase can change. I was after this from many days and today only he accepted it.

My last question that I posted today that if the phases of waves from P(at t=0,x=0) and Q(at t=0,x=0) has a physical importance to answer, what actually was Young controlling (to get a constant phase); the changing frequency(with time) of wave from a single point of main source or that he could not use a single source(monochromatic) as coherent source because the different points on the source(at t=0,x=0)have different phases(the reason which I felt). According to me if at the different points say P and Q of the (monochromatic) source he could get waves with same phase(at t=0,x=0) he would let the interference happen with only one source since such a hypothesis that phases of P and Q of, (monochromatic) source will be same(at t=0,x=0) will about to talk the single source as being coherent source. In that case he won't need two slits. If at any time the waves from different points of sources emerges with common same phase.

You are being too literal here. Note that the phrase "Locked the Phases" is in inverted commas. What it means is that the wave trains from the original source are long enough that, when they reach the two slits, they have traveled near enough the same distance to behave as if they were "locked". What "locking" mechanism could Young have possibly used in the 18th Century? He was just relying on the relatively high level of coherence between the waves on the two paths to the slits. If he had illuminated the two slits by offsetting the light source to one side, the coherence would have deteriorated because the beginnings and ends of the wave trains would no longer coincide at the slits.
In non-laser sources, we are dealing with a large number of atoms that emit their photons in no particular phase relative to each other. That means that the coherence length is very short and you have to use symmetry to help you.
You are applying the 'rules' of discussing ideal theoretical sources with the realities of using simple (ancient) sources. The reasoning behind the simple treatment of Young's Slits is flawed, in fact, if you try to apply it to the real world. But the same goes for all simple electrical and mechanical treatments and you need to approach these things from a mature standpoint. It is fatal to try to 'classify' to slavishly in these things.

In the Young's experiment, there is only one source and what Young was really dealing with was the diffraction pattern of all three slits. It just so happens that you can use symmetry to simplify it down to the pattern of the two front slits.

@ ovalis:
http://www.physikdidaktik.uni-karlsruhe.de/publication/Historical_burdens/54_Coherence.pdf which talks around the concept of coherence. It might help you.

ovais said:
Initially it was told to me here that one can't change phase without change in frequency and that phase and frequency can't be changed independently.

I believe what was meant was that changing the frequency causes a phase shift. You can't have a phase shift without changing the frequency of a real source.

ovais said:
According to me if at the different points say P and Q of the (monochromatic) source he could get waves with same phase(at t=0,x=0) he would let the interference happen with only one source since such a hypothesis that phases of P and Q of, (monochromatic) source will be same(at t=0,x=0) will about to talk the single source as being coherent source. In that case he won't need two slits. If at any time the waves from different points of sources emerges with common same phase.

Sure, if P and Q were the only points emitting light. But real sources are extended sources and consist of many points like Q and P, so the slits are needed to get a distinct interference pattern without light from all the other points adding together to blur and destroy the pattern.

Drakkith said:
I believe what was meant was that changing the frequency causes a phase shift. You can't have a phase shift without changing the frequency of a real source.
Sure, if P and Q were the only points emitting light. But real sources are extended sources and consist of many points like Q and P, so the slits are needed to get a distinct interference pattern without light from all the other points adding together to blur and destroy the pattern.
f = dφ/dt in any source. The coherence will affect the Δφ between two sources.
But ovais's ideas seem to be a confusion between theory and reality. A bit more linear book reading would probably help, rather than the (typical of PF) Brownian chain of Q and A approach that some people seem to favour, which so often gets nowhere fast.

Also, it isn't just the spatial distribution of the sources - it's the granular nature of the emissions with many overlapping short bursts and not a CW signal.

ovais said:
I am writing this just for the purpose to get my this thread a meaning in the hope of getting some more help. I am really sorry for my weak mind which still not understood the after so much help :-(
Don't worry. Everyone has a plus point and a minus point. Maybe you are a slow learner. But I am sure once you understand this, you will never forget it. That's a good point of a slow learner.
ovais said:
Ok I will have a background reading for the above. Will you suggest any book on this which talks about this within my level?
I don't know if the books I am going to recommend are in your country. But eBooks wil be available.
1)HC Verma : Concepts of physics
2)Halliday Resnick and Jearl Walker: fundamentals of physics

One more thing @ovais, in wave optics you should know that independent sources can't act as coherent sources. You know the reason. Secondly the purpose of young's experiment is to show diffraction and formation of fringes. For this first go through Huygens principle. Next to through diffraction. It will be there in the books I suggested you. Now come back to young's experiment. Read it again. You don't have to go through complex theories. In the books I specified enough theory is given to improve your concept. Just learn what's in it. Forget everything else.
Once you have gone through YDSE, solve 50 problems in it to increase your understanding.
In Q5: Young used sodium light. The source is still the same sodium. Frequency depends on the source. So frequency is constant. Now I want you to read about diffraction and think why the slits act as coherent sources. If you know it, its fine.
When you learn things yourself you will have a better understanding.

In Q5: Young used sodium light. The source is still the same sodium. Frequency depends on the source. So frequency is constant.

A sodium light source is not perfectly chromatic and will not be perfectly coherent. It is better than many other sources at producing a narrow range of frequencies though, which is why Young used it.

Drakkith said:
Sure, if P and Q were the only points emitting light. But real sources are extended sources and consist of many points like Q and P, so the slits are needed to get a distinct interference pattern without light from all the other points adding together to blur and destroy the pattern.

Hmm Drakkith seems to understand what I am looking for. Sophiecentaur as assumes that I am trapped under the game of theory and reality but this is not the case Drakkith has been guiding on this thread from more time and I think he is getting closer to the original problem I am feeling:). Sorry for being late to respond I was bit busy with a work given by my Hod. I now will first analyse the answers people gave here and will then be able to know what I need to know still and post it accordingly:-)

I studied during a while about the subject under thread. And with reference to answer of my question by Drakkith in post 26, as below.
Drakkith said:
Yes, the phase at point Q is the same as the phase at point P.

I am satisfied with the answer that at any time say at t=0,the phase of wave at a point P of the source must be same as the phase of wave at any other point Q, at t=0 as told by Drakkith. But I noticed that it may be the case that at t=0, light source is emanating wave from P and there may be no wave emanating from Q at that time(at t=0) but if at this time wave is emanating from Q, than phase of wave from P and Q must be same at t=0,x=0. In fact any instant the phase of wave from every point on source from which wave is emanating has value 0.

Am I correct Sir?

ovais said:
I am satisfied with the answer that at any time say at t=0,the phase of wave at a point P of the source must be same as the phase of wave at any other point Q, at t=0 as told by Drakkith. But I noticed that it may be the case that at t=0, light source is emanating wave from P and there may be no wave emanating from Q at that time(at t=0) but if at this time wave is emanating from Q, than phase of wave from P and Q must be same at t=0,x=0. In fact any instant the phase of wave from every point on source from which wave is emanating has value 0.

Am I correct Sir?

I don't know. You're mixing up different sources here. If P and Q are points on the surface of the source, then the source either generates a wave at points P and Q with the same phase, or it does not.

What I wanted to say is that, let P and Q be two points on a single source of (monochromatic) light. I believe light from both points P and Q will be emitted, each time their(at P and Q) is light producing mechanism(electron jump/movement etc) happen, wave train (of light wave) is emitted(from P is mechanisms happen at P and at Q if mechanism happen at Q). And there is a possibility that at any time light producing phenomenon may take place at P and not at Q at that instant, rather light producing phenomenon(electron jump/movement) is highly random so wave train may be emitted by point Q also but at later time or it(Q) had emitted a wave train earlier than P does. Below is a picture I draw about this understanding of mine. Every help will be highly appreciated.
Regards

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ovais said:
What I wanted to say is that, let P and Q be two points on a single source of (monochromatic) light. I believe light from both points P and Q will be emitted, each time their(at P and Q) is light producing mechanism(electron jump/movement etc) happen, wave train (of light wave) is emitted(from P is mechanisms happen at P and at Q if mechanism happen at Q). And there is a possibility that at any time light producing phenomenon may take place at P and not at Q at that instant, rather light producing phenomenon(electron jump/movement) is highly random so wave train may be emitted by point Q also but at later time or it(Q) had emitted a wave train earlier than P does. Below is a picture I draw about this understanding of mine. Every help will be highly appreciated.
Regards
But you cannot 'turn on' a LED and expect a photon to be emitted at a fixed time after your turn on time. The only way you can ensure that happens is inside a laser when one photon stimulates an atom to emit its photon. That would be within one source and at a quantum level of things - no electrical switches involved. Actually, you can use light from one laser to cause a second laser to emit light that is coherent to it and, that way, you could arrange for two lasers to be synced together and produce beams with a high level of coherence. (But this point has already been made before, I think).

You have to be careful not to try to impose classical ideas of waves on what photons do. It doesn't work very often.

ovais said:
What I wanted to say is that, let P and Q be two points on a single source of (monochromatic) light. I believe light from both points P and Q will be emitted, each time their(at P and Q) is light producing mechanism(electron jump/movement etc) happen, wave train (of light wave) is emitted(from P is mechanisms happen at P and at Q if mechanism happen at Q). And there is a possibility that at any time light producing phenomenon may take place at P and not at Q at that instant, rather light producing phenomenon(electron jump/movement) is highly random so wave train may be emitted by point Q also but at later time or it(Q) had emitted a wave train earlier than P does. Below is a picture I draw about this understanding of mine. Every help will be highly appreciated.
Regards

Now you're getting into non-ideal light sources and non-classical physics.

sophiecentaur said:
But you cannot 'turn on' a LED and expect a photon to be emitted at a fixed time after your turn on time.

I am not expecting that a photon will be emitted at FIXED time after I turn on light rather I learn that emission of photon from a light source is a random phenomenon, random in the sense that the time duration between emission of two successive photons from a given point also it(randomness here) means(as what I understood and depicted in my earlier post) that their is no surety that any two points of the source at any given time is emitting any photon at the first place.

Is it true(what I mentioned) that their can be a wave train from P(means P is emitting photon) while no wave train from Q at the same time(that is no photon emission from Q at that instant) or my this thinking is wrong rather the whole source is illuminated as result of single photon emission within the bulk of the source causing simultaneous emissions of any points be it P and Q, making both points producing wave train simultaneously. And their is no such that that P is emitting wave train but Q is not.

I am not very good at Physics expecially when certain subjects need to look from different angles in different circumstances. I have what no idea how else I clear my doubts. I was not expecting understanding something about light sources would be so difficult :-( and when to take classical model and when something else.

I don't recommend bringing quantum physics into this. Just stick to classical physics for now and forget about photons.

Drakkith said:
I don't recommend bringing quantum physics into this. Just stick to classical physics for now and forget about photons.

Ok let classical physics be under discussion. And I think classically light was considered only as a wave. Now my question is what does classical physics has to say whether a point(from a light source) emit a continuous wave or some part of wave(wave train) take a time lag again produce wave train. So that their is wave trains separated by some space or their is continues wave all the time from all the points.

Classical physics describes the emission of waves from a source, so it has quite a lot to say about this topic, but I don't know what specific questions you're asking so I can't answer you.

Are you asking what determines whether a source emits a steady monochromatic wave or not?

Also if their are wave trains separated by space, does the spaec(distance) between two consecutive wave trains is fixed or random. Also let's chose another point on this same source is it possible that at that other point their is a space between last emitted wave train and source.
Regards

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Drakkith said:
... but I don't know what specific questions you're asking so I can't answer you.

Are you asking what determines whether a source emits a steady monochromatic wave or not?

By steady here I mean continuous wave without break.

I again wanted to recall my previous question where I asked whether it is possible that we have two SEPARATE SOURCES some how(magically) producing monochromatic light, may not producing coherence because(though) they have a fixed(constant and common) frequency but their phase(at any point in space away from source) may not remain constant with time.

I now started looking that a source(even monochromatic one) may not produce coherence because of not steady or non-constant phase of waves from its different points(say P and Q) at a point X in space.

The reason why light from a monochromatic source(unless one use one single and one double slit arrangements after source as what Young did) will not produce interference pattern(or having constant phase difference, please tell am I correct in consideration that they will not keep a constant phase difference at point say X) is seem to be in fact that wave from source(monochromatic) is not continoues but is forms wave trains with random gap.

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ovais said:

That depends on your source. A perfectly monochromatic source emits a single frequency signal for an infinite amount of time. A non-monochromatic source doesn't. Even starting or stopping the signal means that you have a mix of frequencies, leading to a non-monochromatic source.

ovais said:
I again wanted to recall my previous question where I asked whether it is possible that we have two SEPARATE SOURCES some how(magically) producing monochromatic light, may not producing coherence because(though) they have a fixed(constant and common) frequency but their phase(at any point in space away from source) may not remain constant with time.

The phase difference between the two sources at any point in space remains the same over time, otherwise they are not perfectly coherent. The phase of the combined signals still changes over time. This happens if the two sources do not have exactly the same frequency.

ovais said:
I now started looking that a source(even monochromatic one) may not produce coherence because of not steady or non-constant phase of waves from its different points(say P and Q) at a point X in space.

If it's a monochromatic source, then the phase of the waves from each point changes exactly the same amount per unit of time and the two points are required to have coherence.

ovais said:
The reason why light from a monochromatic source(unless one use one single and one double slit arrangements after source as what Young did) will not produce interference pattern(or having constant phase difference, please tell am I correct in consideration that they will not keep a constant phase difference at point say X) is seem to be in fact that wave from source(monochromatic) is not continoues but is forms wave trains with random gap.

A wavefront which is uninterrupted will produce no interference pattern since it doesn't interfere with itself. If you disrupt the wavefront in such a way as to make it interfere with itself, such as making it go through two slits or a lens, an interference pattern will be seen.

That only applies to a single wavefront though. Having two or more sources will always produce an interference pattern since it is not possible to have two different sources emit waves which are both temporally coherent and spatially coherent with identical phases at all points in space.

Drakkith said:
That depends on your source. A perfectly monochromatic source emits a single frequency signal for an infinite amount of time. A non-monochromatic source doesn't. Even starting or stopping the signal means that you have a mix of frequencies, leading to a non-monochromatic source.

I have no doubt that only a perfectly monochromatic source(which does not exist practically) can produce single frequency and that a non-monochromatic source will give a mix of frequencies, rather we can talk how sharp the frequencies are.

My question is still remaining unanswered that if we talk about a source be it monochromatic or non-monochromatic, will it emit steady waves or wave trains separated by space(distance) between them.

Let me for simplicity restrict to monochromatic source, so it will be a great help if you answer above question for specifically monochromatic source.

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ovais said:
My question is still remaining unanswered that if we talk about a source be it monochromatic or non-monochromatic, will it emit steady waves or wave trains separated by space(distance) between them.

A monochromatic source cannot stop and start, it must emit the signal continuously, so you will only get a continuous wave out of it.

A non-monochromatic source can emit either a continuous wave or an interrupted wave. It depends on your source and how you use it.

ovais said:
Also if their are wave trains separated by space, does the spaec(distance) between two consecutive wave trains is fixed or random. Also let's chose another point on this same source is it possible that at that other point their is a space between last emitted wave train and source.
Regards
The lower source in the picture in this post is not a valid idea in this context. By turning the source on and off, you are modulating it. It is no longer a continuous wave.
My opinion is that you are trying to apply arm waving to a subject that is only properly describable using Maths. An arm waving approach is bound to let you down. You need to start from scratch and allow yourself to be lead through the topic by a suitable textbook, rather than trying to get there by asking random questions. Q and A is a very poor way of learning Physics, Maths and Engineering; it leaves you with far too many loose ends.
It could help if you approach the processes of diffraction and interference by considering radio antennae, rather than quantum-based light sources. Radio antennae are much better behaved because you have better control and knowledge of the phases and frequencies of the sources involved - yet the sums are all identical to the optical case. Coherence length for a radio wave relates to the bandwidth involved. A non-continuous wave train, such as you get from a conventional optical source (a series of random bursts from atoms) can be looked upon as a continuous wave, modulated by a random signal. 'Perfect' interference patterns only occur when your two (or more) signals have a predictable phase relationship. If you use modulated signals, then any difference in the path lengths will upset the pattern. (That's the equivalent of Coherence length). Wide band digital or video signals can have a phase that is waving about all over the place, as the modulation drives it. They will not produce neat interference patterns if the path length delays are big enough to upset the relative phases. There are many applications where the bandwidth of the signal (affecting its coherence) will upset the carefully designed off-beam pattern of a directional array. So coherence problems need not only be caused by many different sources (atoms) producing a wave - they can also be caused by modulation.

Drakkith
Drakkith said:
A monochromatic source cannot stop and start, it must emit the signal continuously, so you will only get a continuous wave out of it.

A non-monochromatic source can emit either a continuous wave or an interrupted wave. It depends on your source and how you use it.
OK, wow this is what I wanted to know.
Thank you all

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