What is Coherence? Explained Clearly

[ShayanJ]

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.

 

[ShayanJ]

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]

The simplest coherence relationships occur when all of the waves are traveling in the same direction, and are in phase ... just like a platoon of soldiers marching together. Incoherence is a mob running in all directions.

Laser beams have very strong temporal coherence due to the stimulated emission properties: in phase, same energy, and same direction. Light from very distant sources is also coherent: from stars or the sun. This is because all of the incoherence is lost along the way ... imagine an alley at some distance from the rapidly dispersing mob: those who arrive there will all be going in the same direction!

 

[pmacias]

Coherence refers to the degree of correlation or relationship between two or more waves. It is a measure of how well the waves are synchronized in terms of their frequency, phase, and amplitude. In simpler terms, coherence describes how well waves are "in sync" with each other.

One way to think about coherence is to imagine two people swinging on a swing set. If they are both swinging at the same frequency and in the same direction, they are said to be coherent. However, if one person is swinging faster or in a different direction, they are not coherent.

In terms of waves, coherence is important because it allows us to understand and predict how waves will interact with each other. When two coherent waves meet, they can either amplify or cancel each other out depending on their relative phases. This phenomenon is known as interference and is a key concept in many areas of science and technology.

There are different types of coherence, including temporal coherence (related to the timing of waves), spatial coherence (related to the location of waves), and spectral coherence (related to the frequency of waves). Temporal coherence is often used to describe the behavior of light, while spatial coherence is important in fields such as acoustics and radar.

In summary, coherence is a measure of how well waves are synchronized and can help us understand the behavior of waves in different situations. I hope this explanation helps clarify the concept of coherence for you.