# Interference pattern from a streetlight ?

## Main Question or Discussion Point

If I use a narrow slit (e.g. two fingers) to view a distant streetlight, I can see an interference pattern parallel to the slit. Doesn't this mean that the photons passing through the slit must be phase coherent? If I am correct about coherence, then how does it occur? The photons leaving the streetlight are not coherent; are they?

Excuse me if this question is stupid. I am just an old fuddy-duddy who just likes to think about physics.

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Claude Bile
If the source is sufficiently narrow-band (like a sodium lamp for example) and you are far enough away from the source, then it is possible that the light when it reaches your aperture could be spatially coherent enough to produce an interference pattern.

Spatial coherence is simply a measure of how much the phase varies across a wavefront - it doesn't make a whole lot of sense to label photons as being coherent or incoherent.

Claude.

photons and phase

Perhaps coherence of photons is not the correct term.
The following implies no knowledge of QED, only a reading of Feynmann's "QED" which was intended for the layman.

I thought the angle of the spinning vectors used by Feynmann in "QED" implies an initial phase relationship between photons in deriving an interference pattern. Even if the term is not "coherence" isn't there an assumption in his computations that the pattern is due to the angles of the spinning vectors when they are added at a receive point. IE., in order to make this calculation and generate a "classical" interference pattern, you start by assuming the vectors are aligned (in-phase) at the source point. Then the final vectors are computed based on the possible "paths" of each photon to the receiver.

Andy Resnick
I think what you are seeing is diffraction, not interference.

I don't think so

I think what you are seeing is diffraction, not interference.
Isn't a diffraction pattern the result of constructive and destructive interference?

clem
The diffraction pattern arises from the interference of a photon with itself.
The pattern is found when one photon at a time passes the grating,
after a large number of photons have landed.
Separate photons from the light are not phase coherent.

I am sure that is right.............thank

The diffraction pattern arises from the interference of a photon with itself.
The pattern is found when one photon at a time passes the grating,
after a large number of photons have landed.
Separate photons from the light are not phase coherent.
I am sure that is right.............thanks

I am not sure I posted this in the correct forum..........should have been under quantum.

Claude Bile
I thought the angle of the spinning vectors used by Feynmann in "QED" implies an initial phase relationship between photons in deriving an interference pattern. Even if the term is not "coherence" isn't there an assumption in his computations that the pattern is due to the angles of the spinning vectors when they are added at a receive point. IE., in order to make this calculation and generate a "classical" interference pattern, you start by assuming the vectors are aligned (in-phase) at the source point. Then the final vectors are computed based on the possible "paths" of each photon to the receiver.
I know this is going to sounds picky, but the reasons why I object to the terms "coherent photons" and "incoherent photons" is as follows...
- You can have spatial coherence and temporal coherence. Waves might be spatially coherent but not temporally coherent and vice-versa.
- Waves gain and lose coherence based on the system - it is a dynamic quantity, not a static one.
- Coherence is not a binary quantity, but a continuous one.

It's just one of those things that tends to lead to confusion, which is why I try to avoid labeling waves in this way. Adding photons to the mix further compounds the problem - diffraction can be adequately explained by classical theory without resorting to QM.

Your understanding (and Feynmann's layman explanation as you have presented it) is accurate. Note though how the phase relation of each photon is explicitly stated in time and space, which is the correct way to go about describing coherence!

clem said:
Separate photons from the light are not phase coherent.
This is not correct. You can "enforce" spatial coherence by only permitting a small portion of the wavefront to enter your detector (i.e. through the use of a slit or pinhole). In fact, this is what Young did in his famous experiments.

mangurian said:
I am not sure I posted this in the correct forum..........should have been under quantum.
Nope, this is the right forum .

Claude.

clem
"Separate photons from the light are not phase coherent."

"This is not correct. You can "enforce" spatial coherence by only permitting a small portion of the wavefront to enter your detector (i.e. through the use of a slit or pinhole). In fact, this is what Young did in his famous experiments."

I will not say the you are not correct, but only that my statement is correct.
If Young's slit or pinhole were as small as you suggest, it would have to be of the order of Angstroms (which he couldn't have done) and would produce single slit diffraction.
The correct statement is
Separate photons from the light in Young's experiment are not phase coherent.

Claude Bile
Separate photons from the light in Young's experiment are not phase coherent.
There must be some degree of coherence at the screen for an interference pattern to be produced.

Claude.

pam
A photon interferes with itself, just like an electron in a two slit experiment.

Claude Bile
I can appreciate that fact pam, but going down that path just rephrases the issue in terms of the coherence of the photonic wavefunction, rather than a classical field.

I don't think there is any insight to be gained by bringing photons into the discussion when discussing well understood classical concepts such as single-slit and two-slit diffraction.

Claude.

pam
Spatial coherence is simply a measure of how much the phase varies across a wavefront - it doesn't make a whole lot of sense to label photons as being coherent or incoherent.

Claude.
You have a lot to learn.

Claude Bile
You have a lot to learn.
Well spotted.

Of course you don't get phase variations over a wavefront, since they are lines of equal phase.

What I meant to say is phase variations over the face of a surface such as an aperture.

Claude.

- You can have spatial coherence and temporal coherence. Waves might be spatially coherent but not temporally coherent and vice-versa.

Claude.
Why spatial (transverse) coherence cannot exist with temporal coherence? If light is temporally coherent or that is to say it is monochromatic, I don't see why it cannot be spatially coherent.

Isn't for example laser light temporally coherent and spatially coherent as well as phase coherent.

pam
Isn't for example laser light temporally coherent and spatially coherent as well as phase coherent.
But a street light is not a laser.

But a street light is not a laser.
Of course it is not. But I got the impression that Claude Bile was talking generally of light waves. But why streetlight cannot have temporal and spatial transverse coherence at the same time?

Moreover, how we can have spatial coherence if we do not have temporal coherence in other words monochromatic source of light?!

I know this is going to sounds picky, but the reasons why I object to the terms "coherent photons" and "incoherent photons" is as follows...
- You can have spatial coherence and temporal coherence. Waves might be spatially coherent but not temporally coherent and vice-versa.
- Waves gain and lose coherence based on the system - it is a dynamic quantity, not a static one.
- Coherence is not a binary quantity, but a continuous one.
Because Claude hasn't answered. Could someone please explain why streetlight cannot have spatial and temporal coherence same time, only one or the other?

Andy Resnick