Thin film interference
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Colours in soap bubbles and oil films.
2nt = mλLight reflected from the two surfaces of a film interferes; the condition depends on thickness and phase changes on reflection. Soap bubbles, oil slicks and anti-reflective coatings.
A coating of thickness λ/4n on glass cancels reflection.
Count the π phase changes.
In more detail
Thin films produce colours by interference of light reflected from their top and bottom surfaces. The path difference depends on thickness and refractive index, producing colours such as in soap bubbles and oil slicks.
Real-world uses
- Anti-reflection coatings on lenses use thin-film interference.
- Soap bubbles and oil on water show coloured patterns.
- Soap bubbles and oil films show colours because light reflected from the top and bottom surfaces interferes.
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Frequently asked questions
Why do soap bubbles show colours?
The film thickness varies, so different colours interfere constructively at different places.
What is an anti-reflection coating?
A thin film chosen so reflected waves cancel, reducing reflection.
Why does the colour change as the film thins?
The path difference changes with thickness, so different colours reinforce at different places across the film.
Related topics in Optics
Young's double slit · Fringe width · Intensity in interference · Newton's rings · Michelson interferometer · Single slit diffraction