In Newton's rings, a curved watch glass sits on top of a flat piece of glass. A thin film of air is in between the two - the thin film has a thickness that is zero where the two pieces of glass touch, and gradually increases as you move away from that point.

Light of a single wavelength shines down from above. Light reflecting from the top and bottom surfaces of the thin film interfere, leading to a bulls-eye pattern, as we see on the left (the overhead view). On the right is the side view. Note that the thickness of the thin film is not shown to the same scale as the wavelength of the two reflected waves - those waves show the interference, which can be either constructive or destructive.

Use the sliders to change the wavelength, and the point where we consider the interference in the film.

How does that affect the pattern?

Historical note - Newton's rings were analyzed by Isaac Newton, who was a proponent of the particle theory of light. The phenomenon of Newton's rings, however, provide clear evidence of the wave nature of light.