Thin-Film InterferenceWhy a soap bubble runs with colour. Light bounces off both the front and back of the film; the two reflections are a wavelength or two out of step, so some colours cancel and others add — and the answer shifts as the film thins under gravity. Where it gets thinner than light itself, it goes black.Robert Hooke and Newton catalogued these “Newton’s colours” in the 1670s and Newton used them to argue light was particles. Thomas Young turned the same fringes against him in 1801: only overlapping waves can cancel like this, and from the colours he made the first real measurement of the wavelength of light.