# What is Spatial and Temporal Coherence.

1. May 15, 2013

### onelastdance

Can anyone provide a simple explanation of spatial and temporal. I can't seem to understand the Wikipedia page on Coherence.

2. May 15, 2013

### Staff: Mentor

The simplest way to understand the coherence of a wave is by considering two points in space or in time, dependng on if you are considering spatial or temporal coherence.

If you know the value of the amplitude of a wave at point A, what do you know about the amplitude at point B? If knowing the value at A tells you exactly the value at B, then it is coherent (coherence = 1). If knowing the value at A tells you nothing about the value at B, then it is incoherent (coherence = 0). If knowing the value at A tells you the value at B with a certain uncertainty, then you are somewhere inbetween (0 < coherence < 1), with the smaller uncertainty corresponding to a greater degree of coherence.

Hope this helps.

3. May 15, 2013

### onelastdance

Yes, but what is the difference between spatial and temporal coherence?

4. May 15, 2013

### Staff: Mentor

For spatial coherence, you consider two points in space, for temporal coherence, two distinct instants.

5. May 15, 2013

### Ibix

Spatial coherence is coherence across the beam; temporal coherence is coherence along the beam.

The nomenclature comes from thinking about shining a laser onto a screen. Measure the phase of the wave at one point on the screen. If you've got a spatially coherent source, you can deduce the phase elsewhere in the patch (approximately, within a circle whose radius is the coherence length). On the other hand, if you've got a temporally coherent source, you can calculate the phase at the same point some time in the future (approximately, the time limit for this is the coherence time).

Does that clear up your confusion?