Concentric colored rings, which appear when two pieces of glass are pressed together, are commonly referred to as "Newton Rings". This phenomenon was discoverd by an English physicist Sir Isaac Newton (1643-1727) from which it got its name

The rings appear when there's a tiny air gap between two pieces of clear material. Light rays encounter destructive interference if the gap is of a certain size relative to the wavelength, resulting in the effect. It's most pronounced if a convex lens is pressed up against a perfectly flat glass surface.

Glass which has been treated usually roughened very slightly to prevent Newton's rings from forming is commonly called Anti-Newton glass. It is most often used with film and negative carriers in photographic enlargers. The roughening prevents the formation of tiny air gaps between the surfaces.