What are the two types of coherence?

Coherence describes the properties of the interrelation between physical quantities of a single wave or between several waves.

Two waves are coherent when they have a constant relative phase or when they have zero or constant phase difference and the same frequency.

The two types of coherence:

1. Temporal coherence:
   1.1 Temporal coherence is a measure of the average relationship between the value of a wave and itself making a delay of $\mathrm{\tau }$, at any considerable pair of times.
   1.2 Temporal coherence gives the measurement of how monochromatic a source is. It characterizes how good a wave can interfere with itself at a different time.
2. Spatial coherence:
   2.1 Spatial coherence can be described as the cross-relation between two points in a wave at all times.
   2.2 Spatial coherence can, therefore, be described as the ability of two points in space of a wave that will interfere.