I'm trying to understand Newton's rings. So we have a plano-convex lens supported in a plane (please, see image here http://scienceworld.wolfram.com/physics/NewtonsRings.html). The incident light is divided into the light that is reflected at the convex surface and the light that is reflected at the supporting plane. These two waves interfere, at least close to the support point. Now what I want to understand is why we ignore part that is reflected at the plane surface of the lens. I think it has something to do with coherence. But I can't figure it out. If someone could explain this to me in clear terms, I would very much appreciate it. Thanks.