Newton's Rings
Definition
Rainbow rings where a lens touches glass — thin-film interference made visible, used to test lens perfection.
Technical definition
An air film of varying thickness t sits between lens and plate; dark rings appear where 2t = mλ (with one phase reversal). Ring radii reveal λ or lens curvature.
Example
Opticians check lens smoothness with Newton's rings — distorted rings mean a distorted lens, measurable to fractions of a wavelength.
Common confusion
The rings live in the thin AIR film, not in the glass. Press the lens tighter and the rings expand outward as the film thins.