Thin Film Interference Calculator
Calculate which visible wavelengths are reinforced or cancelled by thin film interference.
Explains soap bubble colors, oil slick iridescence, and AR coatings.
When light reflects off both surfaces of a thin transparent film, the two reflected beams interfere. Whether they reinforce or cancel depends on the film thickness and refractive index.
For a film with n > surrounding medium (e.g., soap in air):
Constructive (bright) interference: 2nt = (m + ½)λ → λ = 2nt/(m + ½) for m = 0, 1, 2…
Destructive (dark) interference: 2nt = mλ → λ = 2nt/m for m = 1, 2, 3…
Where:
- n = refractive index of the film
- t = film thickness (nm)
- λ = wavelength in air (nm)
- m = integer order
The key physics: When light reflects off a surface going from low-n to high-n, it gains a phase shift of 180° (half wavelength). This shifts the interference conditions.
Real-world examples:
- Soap bubbles: As the bubble wall thins near the top, different colors interfere constructively at different thicknesses — creating rainbow swirls
- Oil slicks on water: Oil (n≈1.47) on water (n≈1.33) — similar phase conditions
- Camera lens coatings: A thin MgF₂ coating (n=1.38, t≈100 nm) cancels reflections at green wavelengths → lens appears purple (red+blue reflected)
- CD/DVD rainbow colors: Diffraction rather than thin film, but similar visual effect
- Butterfly wings: Nano-scale thin-film structures create iridescent structural colors without pigment