Thin films show colours due to a fascinating optical phenomenon called thin-film interference. This is the same effect that produces the rainbow patterns on soap bubbles, oil slicks on puddles, and the shimmering iridescence on butterfly wings or peacock feathers.
The Physics Behind It
Wavelength-Dependent (Colour-Dependent) Effects
Here's the key: the amount of interference depends on the wavelength of light (i.e., its colour). For a film of a given thickness:
- Some colours (wavelengths) experience constructive interference → they appear vivid
- Other colours experience destructive interference → they appear washed out or invisible
The condition for constructive interference is roughly:
where:
- = thickness of the film
- = refractive index of the film material
- = wavelength of light
- = an integer (order of interference)
So a film of a particular thickness "selects" certain colours to brighten and others to suppress — giving it a specific colour.
Real-World Examples
- 🫧 Soap bubbles — swirling rainbow colours as the soapy water thins
- ⛽ Oil slicks on wet pavement — colourful patterns from the thin oil layer on water
- 🦋 Butterfly and moth wings — iridescent colours from microscopic layered structures
- 🦚 Peacock feathers — colours produced by thin-film layers, not pigments
- 🛡️ Anti-reflective coatings on glasses and camera lenses — designed so destructive interference cancels reflected light (which is why they look slightly blue or purple)