--- title: "Thin-Film Interference and Anti-Reflection Coatings" description: "Light reflected from the two faces of a thin film interferes, and whether the result is bright or dark depends on the film thickness and on a phase change at reflection. Lens coatings are that conditi" canonical: https://lightmysky.com/learn/science/thin-film-interference-and-anti-reflection-coatings-mt_3ugW-ZijzH source: https://lightmysky.com/learn/science/thin-film-interference-and-anti-reflection-coatings-mt_3ugW-ZijzH.md retrieved: 2026-09-12 --- > **Agent view.** This is the Markdown twin of the page, for tools and assistants. > When to use this site, and the call that answers each job: https://lightmysky.com/agent-instructions.md > API description (OpenAPI 3.1): https://lightmysky.com/openapi.json · Authentication: https://lightmysky.com/auth.md > Pricing: https://lightmysky.com/pricing.md · Catalog: https://lightmysky.com/llms.txt · Full catalog: https://lightmysky.com/llms-full.txt > Every machine-readable file on this domain: https://lightmysky.com/.well-known/ai-catalog.json > Ask for Markdown with `Accept: text/markdown`, a `.md` address, or `?mode=agent`. # Thin-Film Interference and Anti-Reflection Coatings Light reflected from the two faces of a thin film interferes, and whether the result is bright or dark depends on the film thickness and on a phase change at reflection. Lens coatings are that condition engineered on purpose. Subject: Science · Area: Waves, Light & Sound · Ages 19 to 21 Page: https://lightmysky.com/learn/science/thin-film-interference-and-anti-reflection-coatings-mt_3ugW-ZijzH ## Ready when they can - Works out the condition for constructive interference in a film, including the reflection phase change - Explains the colours of an oil film and of a soap bubble as it drains - Designs the thickness of an anti-reflection coating for a stated wavelength ## Lesson: Colours from a thin skin of light Light bounces off both faces of a thin film, and the two reflections interfere. A bounce off a higher index material flips the wave by half a cycle, while a bounce off a lower index one flips nothing. That half flip decides whether the round trip adds up or cancels. A soap film in air flips at the front face only. Head on, bright films satisfy 2 n t equals m plus one half times the wavelength, and dark films satisfy 2 n t equals m times the wavelength. A very thin film goes dark at every colour, and as a bubble drains its thickness sweeps the bright condition across colours in turn. **Example.** A lens coating of magnesium fluoride sits on glass, and both reflections flip, so cancelling needs 2 t to equal half a wavelength inside the film. For 500 nm light and index 1.38 the thinnest layer is 500 divided by 4 times 1.38, which is 90.6 nm. Other colours are only partly cancelled. **Tip.** To design a coating, pick the wavelength you most want to kill and make the layer a quarter of its wavelength inside the film. Quarter wave in, reflection out. **Recap.** Track the two reflections and their flips, then choose the thickness so the target colour cancels. ## Practice 8 questions on this page, each with its working shown. ## Needs first - [Phasor Addition and the Mathematics of Interference](https://lightmysky.com/learn/science/phasor-addition-and-the-mathematics-of-interference-mt_dxh7HHbIUw) ## Opens up - [The Michelson Interferometer](https://lightmysky.com/learn/science/the-michelson-interferometer-mt_Wlb1tZnJBi)