Thin Film Interference Calculator
Calculate which visible wavelengths are reinforced or cancelled by thin film interference.
Explains soap bubble colors, oil slick iridescence, and anti-reflection 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