Coherence length

Coherence length is a measure of how long a light signal stays "in sync" or looks the same. Let's imagine a big line of people all holding sparklers. They all light their sparklers, and they all start making the same shapes in the air with them. As time goes on, the shapes the sparklers make become more and more different from each other, until the shapes look totally random. This is like coherence length – it's a measure of how long the sparklers looked in sync, or kept the same pattern.