Soap Film Interference Lab: Thickness, Drainage & Color (2D)

Film Optics

Refractive index n 1.330

Average thickness 450 nm

Viewing angle (from normal) 15°

Gravity Drainage

Drainage rate 0.80

Sample height (top→bottom) 50%

Live Readout

Reflected color @ sample height

#000000

Top thickness

0 nm

Bottom thickness

0 nm

Sample thickness

0 nm

Elapsed drain time

0.0 s

Film state

draining

How it works

Light reflects off both faces of the soap film. The two reflected rays interfere; their optical path difference for near-normal viewing is

θt = asin( sin(θ_view) / n )
Δ  = 2·n·t·cos(θt)
δ  = (2π/λ)·Δ + π   (the +π is the half-wave phase flip:
                      only the outer air→film reflection flips
                      phase, the inner film→air one does not)
I(λ) = ½·(1 + cos δ)

I(λ) is evaluated independently for the red (620–700 nm), green (500–570 nm) and blue (440–490 nm) bands and used directly as that channel's reflected intensity — the displayed color is the sum of three real interference calculations, not a stock rainbow gradient. When t→0, δ→π for every λ, so all three channels go dark together: that is exactly why a film about to rupture looks black (Newton's "black film"), not colorless-transparent.

Thickness varies with height because gravity drains liquid out of the top of the film into the bottom (and eventually the border) — a well-documented soap-film phenomenon. Here t(y) = t_avg·(0.12+1.7·y)·e^(−drain·τ/45), y=0 at the top. Raising the drainage rate thins the top faster; once it drops under ~3 nm the film is optically black there and the simulation regenerates it (fresh solution flows back up via Marangoni/marginal-regeneration flow in real films).

Bubble view — cross-section colored band-by-band from the live interference calculation.
Spectrum plot — I(λ) traced across the visible range at the sampled height.
Thickness profile — t(y) from top to bottom at the current drain time.

Bubble cross-section Reflectance spectrum I(λ) Thickness profile t(y)