I know this is a very trivial concept, so can someone please point out to me where I'm going wrong with this? We use coherent waves to observe an interference pattern, and coherence by definition is the presence of a constant phase difference between two waves. Yet, we see bright and dark fringes, and at each fringe the phase difference between the two waves is different. Doesn't this contradict the definition of coherence? Where am I getting this wrong? By the way, I'm still in high school, so I would not be able to understand any discussions regarding wave functions and quantum mechanics.