What is Coherence?

Coherence describes how the relative phases of waves behave. If the relative phase is constant, the waves are said to be coherent. If the relative phase changes, the waves are said to be incoherent. When talking about distance, it is called spacial coherence. Spacially coherent waves will travel in the same direction with the same relative phase for long distances, leading to a beam that remains narrow over long distances and that can be focused to a small spot. When talking about time, it is called temporal coherence. Temporally coherent waves will all have the same frequency and thus the same wavelength and energy.