Thin Film Interference Calculator
The shimmering colours of soap bubbles, oil slicks and anti-reflection lens coatings come from light interfering after reflecting off the two faces of a thin transparent film. For a film with a single half-wave phase flip on reflection, this calculator finds which wavelengths are reinforced (seen brightly) and which are cancelled in the reflected light, for a chosen interference order. Enter the film refractive index, its thickness in nanometres and the order m. It assumes normal incidence and an air-film-air style stack with one phase inversion.
Thin film interference formulas
With one phase inversion, normal incidence:
Constructive (bright): lambda = 2 * n * t / (m + 0.5)
Destructive (dark): lambda = 2 * n * t / m
n = film index, t = thickness, m = order
The constructive wavelength is the colour strongly reflected; the destructive wavelength is suppressed. For order m = 0 there is no finite destructive wavelength, so that output reads n/a.
Thin film context
- A soap film in air has a refractive index of about 1.33.
- Anti-reflection coatings are made one quarter wavelength thick to cancel a target colour.
- A reflection off a low-to-high index boundary flips phase by half a wavelength.
- Tilting your view changes the path length, shifting the colours seen on a bubble.
- Visible light spans roughly 380 to 750 nanometres; results in that band are the visible colours.
Thin film interference: frequently asked questions
What causes thin film interference?
Light reflecting off the top and bottom surfaces of a thin transparent film recombines and interferes. Depending on the film thickness and the wavelength, some colours reinforce and others cancel, producing the shifting colours seen on soap bubbles, oil slicks and anti-reflection coatings.
What is the thin film interference formula?
For a film with one phase inversion on reflection (a low-high-low index stack such as air-film-air), constructive interference in reflection occurs at 2 * n * t = (m + 0.5) * lambda and destructive at 2 * n * t = m * lambda, where n is the film index, t the thickness and m a non-negative integer order.
Why does a half-wavelength phase shift appear?
Light reflecting from a boundary where the index increases (low to high) flips phase by half a wavelength; reflecting from a high-to-low boundary it does not. For a soap film in air only the top reflection flips, so a single half-wave shift is built into the condition. Stacks with two flips swap the constructive and destructive formulas.
What refractive index should I use?
Use the index of the film material at the wavelength of interest. Soap or water films are about 1.33, oil films around 1.45, and magnesium fluoride coatings about 1.38. Refractive index is a measured material property, so it ships here as a user-editable input.
Does this assume light hits the film straight on?
Yes. The formula here is for normal incidence, where light strikes the film perpendicular to its surface. At an angle the effective path length changes and a cosine factor of the refraction angle is needed, which shifts the colours, the reason a bubble's colours move as you tilt your view.
Official sources
- U.S. National Institute of Standards and Technology: Physical Measurement Laboratory.
- NASA Science: Wave behaviors of light.
Reviewed by the CalculatorHub team, edited by James Graham, 16 June 2026. See our methodology.