Newton's rings

Newton's rings are concentric bright and dark fringes produced by interference between light reflected from the curved surface of a lens and a flat plate, with a thin air film between them.

Thin-film interference: Interference between light waves reflected from the boundaries of a thin layer, producing wavelength-dependent bright and dark regions. The changing air-film thickness between lens and plate creates the alternating fringes.
Interference: The redistribution of wave amplitude when two or more waves overlap. Constructive and destructive interference create the bright and dark rings.
Wavelength measurement: The determination of a wave's spatial period from observations or instrument readings. Known lens curvature and measured ring diameters yield the light's wavelength.
Isaac Newton: An English mathematician and natural philosopher whose work shaped classical mechanics, optics, and mathematics. The circular fringes became associated with his investigations of light and thin films.
Newton's rings experiment: An optical experiment in which a curved lens rests on a flat plate to produce concentric interference fringes. The standard arrangement makes the air film and ring geometry directly observable.
Optical path length: The distance light effectively travels through a medium, equal to geometric distance multiplied by refractive index. It determines the phase difference between the two reflected waves.
Reflection: The return of a wave from a boundary between materials into the medium from which it came. The two relevant reflected waves come from opposite boundaries of the air film.
Lens testing: The evaluation of a lens's optical properties, surface figure, and defects. Departures from ideal ring shapes can reveal surface irregularities or poor contact.
Opticks: Isaac Newton's 1704 book presenting experiments and theories on light, color, and optical phenomena. Newton described the rings and reported measurements of their dimensions.
Reflected-light Newton's rings: Newton's rings observed in light reflected from the upper and lower boundaries of a thin film. This is the usual configuration, with a dark central spot under ideal contact and monochromatic illumination.

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