Understanding Coherent Length & Interference Effects

[mysqlpress]

What is coherent length?
and how it is affected interference

 

[Claude Bile]

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.

 

[lightarrow]

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.

 

[mysqlpress]

Thanks. but usually what is the order of magnitude of coherent length as well as coherent time?

 

[Andy Resnick]

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.