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

#### Drakkith

Staff Emeritus
2018 Award
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.

#### ovais

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:-)

#### ovais

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.
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?

#### Drakkith

Staff Emeritus
2018 Award
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.

#### ovais

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

#### Attachments

• 35.7 KB Views: 191

#### sophiecentaur

Gold Member
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.

#### Drakkith

Staff Emeritus
2018 Award
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.

#### ovais

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.

#### Drakkith

Staff Emeritus
2018 Award
I don't recommend bringing quantum physics into this. Just stick to classical physics for now and forget about photons.

#### ovais

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.

#### Drakkith

Staff Emeritus
2018 Award
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?

#### ovais

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

#### Attachments

• 34.6 KB Views: 178

#### ovais

... 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.

#### ovais

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.

#### Attachments

• 20.5 KB Views: 161

#### Drakkith

Staff Emeritus
2018 Award
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 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.

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.

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.

#### ovais

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.

Last edited:

#### Drakkith

Staff Emeritus
2018 Award
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.

#### sophiecentaur

Gold Member
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 text book, 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.

#### ovais

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

### Physics Forums Values

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving