Why does a thin oil film on water show rainbow colours?

Light reflecting from the top and bottom of the film interferes; the path difference depends on film thickness and viewing angle, so different wavelengths are reinforced at different places, producing shifting bands of colour.

How does an anti-reflection coating work?

A coating one quarter-wavelength thick with an intermediate refractive index makes the reflections from its two surfaces interfere destructively, cancelling much of the reflected light and increasing transmission.

Thin Films and Interferometry

Thin-film interference and interferometers turn optical path differences into measurable fringes, used for coatings and for precise measurement of length and shape.

Definition

The use of interference between waves reflected from the surfaces of thin layers, or between beams in an interferometer, to produce fringe patterns whose changes reveal optical path differences with high precision.

Scope

This topic covers interference produced by division of amplitude in thin films and by interferometers. It includes the colours of soap films and oil slicks, the conditions for constructive and destructive interference in a single film, anti-reflection and high-reflection multilayer coatings, and the principles of the Michelson, Mach-Zehnder, Sagnac, and Fabry-Perot interferometers. It treats how interferometers convert minute changes in path length into shifts of fringes for metrology, spectroscopy, and the sensing of displacement and refractive index.

Core questions

What determines the colours seen in a thin film such as a soap bubble?
How do multilayer coatings suppress or enhance reflection?
How does an interferometer convert a path-length change into a measurable fringe shift?
What distinguishes the common interferometer configurations?

Key concepts

thin-film interference
anti-reflection coating
phase change on reflection
Michelson interferometer
Mach-Zehnder interferometer
Fabry-Perot etalon
fringe shift
optical metrology

Key theories

Thin-film interference: Light reflected from the front and back surfaces of a thin layer interferes; the path difference, including phase changes on reflection, sets which wavelengths are reinforced, producing the characteristic colours and the basis for optical coatings.
Interferometric measurement: Interferometers split light into two paths and recombine it, so that a change in one path by a fraction of a wavelength shifts the fringes measurably, enabling displacement, surface, and refractive-index metrology.

Clinical relevance

Anti-reflection coatings improve the transmission of lenses in microscopes, endoscopes, and spectacles, while interferometric methods enable precise measurement of corneal topography and axial eye length used in cataract surgery planning.

History

Newton studied the coloured rings of thin films that bear his name, though he interpreted them within his corpuscular framework. The wave explanation followed from Young and Fresnel, and Michelson's interferometer of the 1880s, used in the Michelson-Morley experiment and later to define the metre, established interferometry as a precision tool.

Key figures

Albert A. Michelson
Ludwig Mach
Isaac Newton

Seminal works

hecht2017
bornwolf1999

Frequently asked questions

Why does a thin oil film on water show rainbow colours?: Light reflecting from the top and bottom of the film interferes; the path difference depends on film thickness and viewing angle, so different wavelengths are reinforced at different places, producing shifting bands of colour.
How does an anti-reflection coating work?: A coating one quarter-wavelength thick with an intermediate refractive index makes the reflections from its two surfaces interfere destructively, cancelling much of the reflected light and increasing transmission.