What is coherence?

Gold Member
Recently I've been trying to understand coherence,about waves in general.But its just a little hazy to me.Can someone explain it clearly?
Thanks

Andy Resnick
This question is asked here repeatedly- have you done any searching?

Coherence is a statistical concept that measures how well you can predict results at other times (temporal coherence) and places (spatial coherence), given a single measurement at your current time and place. The 'mutual coherence function' is the most general expression, 'coherence time' and 'coherence area' are restricted to either the time or space domains. Coherence time depends on the (temporal) frequency bandwidth of the source, coherence area depends on the spatial extent of the source.

Gold Member
Well,I guess I understand that.I'm just confused because people say two waves can interfere if they are coherent with each other or sth like this.But I don't understand why.Interference seems to be a local phenomenon i.e. the amplitude and phase of the waves at one point and only at that point affect the result of their interference but coherence is about the connection of two points,in space or time!

UltrafastPED