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Coherent light means monochromatic light and alle waves have the same phase difference. This is given for laser, where the resonator is a potential box and the outgoing waves have the same phase difference. What is about a thermal source like a bulb? The emitting surface is not smooth, the involved electrons are not swinging in phase. So, how it is possible to produce coherent light with a thermal source?

The background for this question: https://physics.stackexchange.com/questions/183935/why-the-distance-between-the-light-source-and-the-slits-screen-seems-to-be-a-po So the question is about coherence in front of any slit or edge.

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HolgerFiedler
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    With double silts – Paul May 14 '15 at 07:06
  • @Paul This I fully agree. It was my failure not to point out the background. I will add this information in the question. – HolgerFiedler May 14 '15 at 07:12
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    You need to distinguish between temporal coherence and spatial coherence. The use of a pinhole creates a spatially coherent source because it samples a small area of the light wavefronts. – John Rennie May 14 '15 at 07:19
  • @JohnRennie Fully agree but this does not solve the problem. I'm asking for the case in front of the first edges (the pinhole). – HolgerFiedler May 14 '15 at 07:30
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    I still don't get what you're asking. Spatial coherence means as we move normal to the direction of propagation the phase remains constant. So if you sample a small (enough) part of a wavefront, as a pinhole does, then the light you get is automatically spatially coherent. However it is not necessarily temporally coherent. – John Rennie May 14 '15 at 07:41
  • @JohnRennie To get spatial coherence behind a pinhole all waves in the line of the opening have to have the same phase difference. We are moving the problem from the slit to the pinhole. The pinhole and the thermal source have to be a potential box. This seems to me not to be possible. – HolgerFiedler May 14 '15 at 07:49
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    Ah, OK, so it sounds as if your question is really How does a pinhole produce spatially coherent light. Is that a fair description? – John Rennie May 14 '15 at 07:51
  • @JohnRennie Yes! Agree with you. And pleas in constellation with http://physics.stackexchange.com/questions/183935/why-the-distance-between-the-light-source-and-the-slits-screen-seems-to-be-a-po – HolgerFiedler May 14 '15 at 08:12
  • I think I must have been twelve when I did the experiment with an aluminum foil pinhole covering a flashlight and a soapy mirror. The interference rings are very pretty. :-) – CuriousOne May 14 '15 at 08:22
  • I don't understand the question. Slits and pinholes are both mentioned. Is this about spatial coherence or temporal (spectrometer with slit)? And I don't know what a potential box is. – garyp May 14 '15 at 14:07
  • @garyp https://en.wikipedia.org/wiki/Particle_in_a_box – HolgerFiedler May 14 '15 at 14:17
  • Ok. I've never heard of a resonator called a potential box. But I still don't understand the question, so I'll step aside. – garyp May 14 '15 at 14:27

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Coherency of light in practice is not an either/or issue. Any light due to any source has some degree of coherence. Laser light has usually much higher coherence than light of a hot metal filament.

Some degree of coherence means, in simple wording, that light waves at one point of space due to different parts of the source behave similarly (they have non-zero covariance). For thermal source, the lower the distance between two of its parts, the greater the covariance of their retarded fields at the observation point.

In usual circumstances the coherence of light due to thermal sources is very low and interference effects are hard to observe. It can be increased, however, with color filter and a diaphragm with a narrow slit/hole placed in the path of the light before it hits the measuring instrument.

Ján Lalinský
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  • Does that mean that a edge or a slit unifying the phases of electromagnetic waves from inherent sources? – HolgerFiedler May 14 '15 at 20:33
  • If more sources are considered and not only total field, it is preferable to think of the diaphragm as additional source. Due to action of the retarded field from the original source, the diaphragm acts as source of EM field too and due to the slit with appropriate shape can make the net EM field on the other side more coherent. – Ján Lalinský May 14 '15 at 20:38
  • Nice, so there is an interaction between the slit - more practicable it seems to me with the surface electrons from the slits material - and the incoming waves. For some reason I prefer to talk about incoming photons instead of EM waves. – HolgerFiedler May 14 '15 at 20:42
  • Or does I missinterpreting your? I want to citating this discussion and have to be sure, that the slits material and the waves are interacting, they influence each other. right or my misinterpretation? – HolgerFiedler May 14 '15 at 20:51
  • Of course, they interact, otherwise the wall would do nothing to the light. The more usual description of the action of the wall with slit is based on theory of diffraction, where the slit is treated merely as geometrical obstacle to light rays. This simplifies mathematics, but physically, the wall interacts with the primary light of the source. – Ján Lalinský May 14 '15 at 20:53
  • Would your time allows you to give an answer to this question – HolgerFiedler May 14 '15 at 21:00