Newton's rings are an interference pattern caused when monochromatic light reflects off a spherical surface, like a lens, and an adjacent flat surface, like a glass sheet. This forms a thin air film between the surfaces that varies in thickness. At points of constructive interference, bright rings appear; at points of destructive interference, dark rings appear. The spacing between the rings gets smaller further from the center. The formula for the radius of the nth bright ring is derived from considering the optical path difference between light reflecting off the two surfaces.