What restricts two separte sources of light to act coherent?

[Drakkith]

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?

 

[Ahsan Khan]

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

 

[Ahsan Khan]

... 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?

I am asking whether a source emits steady(monochromatic/non-monocromatic) wave or non-steady(monochromatic/non-monocromatic) light?

By steady here I mean continuous wave without break.

 

[Ahsan Khan]

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.

 

[Drakkith]

I am asking whether a source emits steady(monochromatic/non-monocromatic) wave or non-steady(monochromatic/non-monocromatic) light?

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.

 

[Ahsan Khan]

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.

 

[Drakkith]

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]

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.

 

[Ahsan Khan]

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