I was just reading my book when it said that the fact that newtons rings were always dark in the center where the lens touches the glass plate is proof that reflections are 180 degrees out of phase with the incident light, but after further thinking, I think that logic doesn't make complete sense. It says there are other experiments that prove reflections are 180 degrees out of phase so I would like it if someone would tell me what they are. The problem with the logic is that if an incident wave reflects first at one surface and then at the other like in thin films, even if the surfaces are closer than a wavelength, they would both be out of phase with the incident light and would therefore be in phase with each other if the surfaces are close enough. The book implies that they should now be out of phase to match the experiment but according to my logic, the reflected waves, should still be in phase with each other. I would also wonder in the experiment if any reflection is taking place in the center anyway. Are some reflections perfectly in phase and other reflections perfectly out of phase with the incident light? Are there other experiments that prove perfectly out of phase reflections?