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Newton's rings are seen whenever two more or less smooth surfaces are close to touching each other, but are not exactly parallel. They're most commonly seen in oil slicks on wet pavement, where you see a rainbow of colors because the light is reflecting back and forth between the two surfaces (air-oil and oil-water). The colors are caused by differences in the thickness of the film. If a film is truly flat, it can act as a color filter; nebula filters are an example, as are antireflection coatings on lenses. You might notice Newton's Rings in an improperly spaced telescope objective lens. They don't affect the image directly, but they do indicate a collimation problem. You would see them by removing the eyepiece and putting your eye at the focus or by looking in from the other end at reflected light.

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