what is "coherent" light? In high school, I learned that coherent light is light which has same frequency,phase and so on. In college , I learn that there is temporal coherent and spatial coherent. why we say laser is coherent source? I am confused. what is coherent? Now the new light source like free electron laser ,we all say it is coherent x ray,what does the mean coherent? what the difference between x ray which generated in x ray tube and free electron laser? Thank you all!