Understanding Coherent Length & Interference Effects

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Discussion Overview

The discussion centers around the concepts of coherence length and coherence time in the context of wave interference. Participants explore the definitions, implications, and relationships between these concepts, particularly in relation to interference patterns and the behavior of waves.

Discussion Character

  • Technical explanation, Conceptual clarification, Debate/contested

Main Points Raised

  • One participant defines coherence time as the duration it takes for different frequency components of a wave to drift out of phase, suggesting that coherent waves exhibit interference patterns while incoherent waves do not.
  • Another participant clarifies that coherence length refers to the spatial extent over which a specific phase relationship among frequencies is maintained, contrasting it with coherence time.
  • A question is raised regarding the typical order of magnitude for coherence length and coherence time, indicating a desire for quantitative context.
  • One participant connects coherence length to the maximum path difference in an unequal-arm interferometer, emphasizing the relationship between coherence length, coherence time, and spectral bandwidth.
  • There is a mention of spatial coherence and its relation to the size of a source, highlighting the distinction between temporal and spatial coherence in discussions.

Areas of Agreement / Disagreement

Participants express differing views on the definitions and implications of coherence length and coherence time, indicating that multiple competing perspectives exist without a clear consensus.

Contextual Notes

Some participants note the importance of distinguishing between temporal and spatial coherence, suggesting that definitions may depend on specific contexts or applications.

Who May Find This Useful

This discussion may be useful for individuals interested in wave physics, optics, and the principles of interference, particularly in academic or experimental settings.

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What is coherent length?
and how it is affected interference
 
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Okay, all real waves have a continuous spectrum of frequencies. Different frequencies drift out of phase with time. The coherence time is the length of time it takes for different frequency components to be out of phase by some amount - typically one eighth or one quarter of a cycle.

Only coherent waves will show an interference pattern. Incoherent waves will not because they don't have well defined maxima and minima.

Claude.
 
Yes, but he asked about coherence "lenght", not "time".
AFAIK, coherence length is the spatial length over which there is a specific phase relation among the different frequencies of the spectrum, while out of that length the phases are casual.
 
Thanks. but usually what is the order of magnitude of coherent length as well as coherent time?
 
The coherence "length" is taken to refer to the maximum path difference in an unequal-arm interferometer, and thus actually refers to the coherence time (L = c*t), which is given by the spectral bandwidth.

The coherence "area" refers to spatial coherence and is related to the apparent size of a source.

That's why I tend to be explicit when discussing this stuff in class- temporal or spatial coherence.
 
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