What factors affect the coherence of waves?

[alchemist7]

I do understand the definition that their initial phase different is not constant. but i just cannot imagine what kind of wave has changing initial phase. isn't initial phase the phase at the time 0? how can it change?

 

[Born2bwire]

Incoherent waves have really bad lisps and they usually stutter.

Coherent waves have constant relative phase if I recall correctly. For most cases this means that the waves are the same frequency. Monochromatic sources like a laser are an example of a coherent source. Shifts in amplitude also introduce incoherence too I think.

 

[Bob S]

An example of an incoherent wave would be the sodium arc lamps used for street lamps that have a doublet yellow line at about 5880 Angstroms. All of the photons are incoherent, having neither constant phase nor exact wavelength as any other photons.

 

[jtbell]

I do understand the definition that their initial phase different is not constant. but i just cannot imagine what kind of wave has changing initial phase.

Suppose you have a bunch of short wavetrains that have finite lengths (and durations), and the same frequency and wavelength, but start at randomly different times. Now superpose them to form a resultant wave.

 

[Andy Resnick]

Coherence (and incoherence) are statistical properties of a collection of oscillators. In highly coherent fields, the relative phase stays constant in space and time. For highly incoherent fields, the relative phase changes rapidly. One can have spatial coherence and temporal coherence, and control them separately.

Standard ways of decreasing the spatial coherence is by transmission or reflection off a moving diffuse surface, or multiple reflections from a highly diffuse surface (integrating sphere). The spatial coherence can be increased by using a spatial filter (pinhole). Temporal coherence can be increased by spectral filters.